A temperature swing adsorption process based on photovoltaic cells

Abstract

The present application relates to the field of photovoltaic cells, and particularly relates to a variable-temperature diffusion process based on photovoltaic cells. Specifically, it comprises the following steps: (1) low-temperature deposition on a silicon wafer with completed tunneling oxide layer and intrinsic amorphous silicon at 760-790 degrees Celsius; (2) low-temperature promotion at a temperature increased by 30-40 degrees Celsius; (3) high-temperature deposition at a temperature increased by 60-70 degrees Celsius; (4) high-temperature promotion at the temperature of step (3); (5) medium-temperature deposition at a temperature decreased by 20-50 degrees Celsius; and (6) low-temperature deposition at a temperature decreased by 80-100 degrees Celsius. The present application reduces the amount of phosphorus elements penetrating the tunneling oxide layer and increases the concentration of phosphorus in the doped polysilicon through gradient temperature rising diffusion, promotion and gradient temperature decreasing diffusion.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cells, and more particularly to a variable temperature diffusion process based on photovoltaic cells. Background Technology

[0002] With the development of the photovoltaic solar energy market, more and more new technologies have emerged. Among them, TOPCon cell technology prepares an ultrathin tunneling oxide layer and a doped polycrystalline silicon layer on the back of the silicon wafer. The two together form a passivation contact structure, which provides a good passivation effect for the back of the silicon wafer. The ultrathin oxide layer allows majority carrier electrons to tunnel through while blocking minority carrier holes from recombinating. Electrons are laterally transported in the polycrystalline silicon layer and collected by the metal, which greatly reduces the metal contact recombination current and improves the open circuit voltage and short circuit current of the cell.

[0003] Because the tunneling oxide layer is relatively thin, phosphorus atoms can penetrate it during the formation of high-concentration phosphorus-doped polycrystalline silicon. This leads to the formation of new recombinations on the side of the oxide layer closer to the silicon wafer, reducing the surface passivation effect. Simultaneously, the phosphorus concentration in the polycrystalline silicon layer decreases accordingly, resulting in poorer contact characteristics with the paste and a deterioration in field passivation. Therefore, controlling the number of phosphorus atoms penetrating the tunneling oxide layer while maintaining a high doping concentration in the polycrystalline silicon is a challenging task.

[0004] In the prior art, CN103943719A discloses a method for controlling phosphorus doping concentration using a pre-oxidation combined with a low-temperature-high-low-temperature diffusion process. This involves first performing low-temperature oxidation and low-temperature deposition diffusion, then high-temperature diffusion and bonding, and finally cooling diffusion and bonding. However, the three bonding steps occur during the diffusion process, making it difficult to control the amount of phosphorus atoms penetrating the tunneling oxide layer, and the gettering effect is limited. CN102383198A discloses a three-step variable-temperature diffusion process for crystalline silicon solar cells. The first step involves low-temperature pre-deposition diffusion, the second step involves heating for bonding, and the third step involves heating again for deposition diffusion. However, for doped polycrystalline silicon structures, the metal ohmic contact effect is reduced, and electron lateral transport also decreases. CN113808927A discloses a phosphorus diffusion process for TOPCon solar cells, but its amorphous layer crystallization time is relatively long, resulting in high manufacturing costs. Furthermore, the diffusion temperatures remain unchanged between the two steps, and whether it effectively prevents phosphorus atoms from penetrating the tunneling oxide layer needs further verification. CN113903833A discloses an LPCVD process for TOPCon batteries. However, this technology reduces the penetration of phosphorus atoms into the tunneling oxide layer by forming amorphous silicon of different densities during the LPCVD process and utilizing the different diffusion rates of phosphorus atoms within it. However, this method requires two different LPCVD deposition temperatures, resulting in a long temperature adjustment time. CN110983289A discloses a method for preparing passivated contact structures based on secondary LPCVD ion implantation. However, this method requires two ion implantations, two cleaning processes, and two annealing processes, making the process complex, time-consuming, and costly. Summary of the Invention

[0005] This invention provides a variable-temperature diffusion process based on photovoltaic cells to solve the above-mentioned defects, reduce the amount of initial phosphorus element penetrating the tunnel oxide layer, and at the same time ensure a high doping concentration in the doped polycrystalline silicon.

[0006] This invention provides a variable-temperature diffusion process based on photovoltaic cells, comprising the following steps:

[0007] (1) Low-temperature deposition of silicon wafers with completed tunnel oxide layer and intrinsic amorphous silicon at 760-790 degrees Celsius;

[0008] (2) Increase the temperature by 30-40 degrees Celsius to conduct low-temperature propulsion;

[0009] (3) Increase the temperature by 60-70 degrees Celsius to perform high-temperature deposition;

[0010] (4) Maintain the temperature of step (3) and carry out high-temperature propulsion;

[0011] (5) Lower the temperature by 20-50 degrees Celsius and perform medium-temperature deposition;

[0012] (6) Lower the temperature by 80 to 100 degrees Celsius and perform low-temperature deposition.

[0013] According to the variable temperature diffusion process based on photovoltaic cells provided by the present invention, steps (1), (3), (5), and (6) are carried out under the condition of introducing small nitrogen gas, oxygen gas, and large nitrogen gas; steps (2) and (4) are carried out under the condition of introducing only large nitrogen gas.

[0014] According to the variable temperature diffusion process based on photovoltaic cells provided by the present invention, in steps (1) and (3), the flow rate of small nitrogen gas is 800-1800 standard milliliters / minute, the flow rate of oxygen is 300-800 standard milliliters / minute, and the flow rate of large nitrogen gas is 500-1000 standard milliliters / minute;

[0015] In steps (2) and (4), the flow rate of nitrogen gas is 1000-2000 standard milliliters / minute;

[0016] In step (5), the flow rate of small nitrogen gas is 800-1800 standard milliliters / minute, the flow rate of oxygen is 300-1200 standard milliliters / minute, and the flow rate of large nitrogen gas is 400-1000 standard milliliters / minute;

[0017] In step (6), the flow rate of small nitrogen gas is 600-1800 standard milliliters / minute, the flow rate of oxygen is 500-3000 standard milliliters / minute, and the flow rate of large nitrogen gas is 600-2000 standard milliliters / minute.

[0018] According to the variable temperature diffusion process based on photovoltaic cells provided by the present invention, in step (1), the low temperature deposition time is 10 to 20 minutes;

[0019] In step (2), the cryogenic propulsion time is 1 to 5 minutes;

[0020] In step (3), the high-temperature deposition time is 2 to 5 minutes;

[0021] In step (4), the high-temperature propulsion time is 8 to 18 minutes;

[0022] In step (5), the medium-temperature deposition time is 2 to 6 minutes;

[0023] In step (6), the low-temperature deposition time is 5 to 10 minutes.

[0024] According to the present invention, a variable temperature diffusion process based on photovoltaic cells is provided, the process further includes oxidizing the silicon wafer with the completed tunnel oxide layer and intrinsic amorphous silicon before step (1); wherein the oxidation conditions are: nitrogen gas and oxygen are introduced at 80-200 Pascal and 760-790 degrees Celsius, the oxidation time is 1-5 minutes; the oxygen flow rate is 300-1000 standard milliliters / minute, and the nitrogen gas flow rate is 500-1500 standard milliliters / minute.

[0025] According to the present invention, a variable temperature diffusion process based on photovoltaic cells is provided, the process further includes oxidizing the silicon wafer after low-temperature deposition after step (6); wherein the oxidation conditions are: nitrogen gas and oxygen are introduced at 80-300 Pascal and 700-730 degrees Celsius, and the oxidation time is 3-7 minutes; the oxygen flow rate is 800-3000 standard milliliters / minute, and the nitrogen gas flow rate is 800-3000 standard milliliters / minute.

[0026] According to the variable temperature diffusion process based on photovoltaic cells provided by the present invention, the heating rate of steps (2) and (3) is 12 to 16 degrees Celsius per minute.

[0027] According to the variable temperature diffusion process based on photovoltaic cells provided by the present invention, the cooling rate of steps (5) and (6) is 3 to 5 degrees Celsius per minute.

[0028] The present invention also provides a silicon wafer, which is prepared by the above-described variable temperature diffusion process.

[0029] The present invention also provides a photovoltaic cell comprising the aforementioned silicon wafer.

[0030] According to the present invention, a photovoltaic cell is a TOPCon cell.

[0031] The present invention also provides the application of the above-mentioned silicon wafer in the field of photovoltaic cells.

[0032] The present invention provides a variable temperature diffusion process based on photovoltaic cells, which reduces the amount of phosphorus that penetrates the tunneling oxide layer by gradient heating diffusion and gradient cooling diffusion, while increasing the phosphorus concentration in doped polycrystalline silicon. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0034] Figure 1This is a schematic diagram of the diffusion temperature curve provided by the present invention;

[0035] Figure 2 This is a flow chart of the diffusion process provided by the present invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0037] The following is combined Figures 1 to 2 The variable-temperature diffusion process based on photovoltaic cells described in this invention includes the following steps:

[0038] (1) Low-temperature deposition of silicon wafers with completed tunnel oxide layer and intrinsic amorphous silicon at 760-790 degrees Celsius;

[0039] (2) Increase the temperature by 30-40 degrees Celsius to conduct low-temperature propulsion;

[0040] (3) Increase the temperature by 60-70 degrees Celsius to perform high-temperature deposition;

[0041] (4) Maintain the temperature of step (3) and carry out high-temperature propulsion;

[0042] (5) Lower the temperature by 20-50 degrees Celsius and perform medium-temperature deposition;

[0043] (6) Lower the temperature by 80 to 100 degrees Celsius and perform low-temperature deposition.

[0044] The present invention found that when low-temperature diffusion and low-temperature propulsion are performed at the temperatures defined in steps (1) and (2) respectively, the amount of phosphorus element passing through the tunneling oxide layer is small, which can reduce the recombination on the silicon wafer surface; while when high-temperature diffusion and high-temperature propulsion are performed at the temperatures defined in steps (3) and (4) respectively, it is beneficial to control the surface doping concentration, ensure that the polycrystalline silicon has a high phosphorus concentration, and enhance the lateral electron transport and subsequent contact characteristics with the paste.

[0045] Further research revealed that gradient cooling diffusion at the temperatures defined in steps (5) and (6) after the gradient heating diffusion can further increase the phosphorus concentration on the polycrystalline silicon surface, thereby improving the field passivation effect and contact characteristics of the polycrystalline silicon.

[0046] This invention relates to a diffusion doping process for a tunneling oxide layer and an intrinsic amorphous silicon layer that have already been completed. In specific implementations, those skilled in the art can use conventional methods in the field, such as PECVD and PVD, to complete the tunneling oxide layer and the intrinsic amorphous silicon layer as needed.

[0047] According to a preferred embodiment of the present invention, steps (1), (3), (5), and (6) are performed under conditions of introducing small nitrogen gas, oxygen gas, and large nitrogen gas; steps (2) and (4) are performed under conditions of introducing only large nitrogen gas.

[0048] According to a preferred embodiment of the present invention, in steps (1) and (3), the flow rate of small nitrogen gas is 800-1800 standard milliliters / minute, the flow rate of oxygen is 300-800 standard milliliters / minute, and the flow rate of large nitrogen gas is 500-1000 standard milliliters / minute;

[0049] In steps (2) and (4), the flow rate of nitrogen gas is 1000-2000 standard milliliters / minute;

[0050] In step (5), the flow rate of small nitrogen gas is 800-1800 standard milliliters / minute, the flow rate of oxygen is 300-1200 standard milliliters / minute, and the flow rate of large nitrogen gas is 400-1000 standard milliliters / minute;

[0051] In step (6), the flow rate of small nitrogen gas is 600-1800 standard milliliters / minute, the flow rate of oxygen is 500-3000 standard milliliters / minute, and the flow rate of large nitrogen gas is 600-2000 standard milliliters / minute.

[0052] According to a preferred embodiment of the present invention, in step (1), the low-temperature deposition time is 10 to 20 minutes;

[0053] In step (2), the cryogenic propulsion time is 1 to 5 minutes;

[0054] In step (3), the high-temperature deposition time is 2 to 5 minutes;

[0055] In step (4), the high-temperature propulsion time is 8 to 18 minutes;

[0056] In step (5), the medium-temperature deposition time is 2 to 6 minutes;

[0057] In step (6), the low-temperature deposition time is 5 to 10 minutes.

[0058] According to a preferred embodiment of the present invention, the process pressure in steps (1) to (5) is 80 to 200 Pascals, and the process pressure in step (6) is 80 to 300 Pascals.

[0059] According to a preferred embodiment of the present invention, the variable temperature diffusion process based on photovoltaic cells further includes oxidizing the silicon wafer with the completed tunnel oxide layer and intrinsic amorphous silicon before step (1); wherein the oxidation conditions are: nitrogen gas and oxygen are introduced at 80-200 Pascal and 760-790 degrees Celsius, and the oxidation time is 1-5 minutes; the oxygen flow rate is 300-1000 standard milliliters / minute, and the nitrogen gas flow rate is 500-1500 standard milliliters / minute.

[0060] The present invention has found that performing low-temperature oxidation at the above-mentioned temperature before diffusion can improve the uniformity of phosphorus impurity diffusion and at the same time act as a barrier layer to effectively control the phosphorus diffusion concentration.

[0061] According to a preferred embodiment of the present invention, the variable temperature diffusion process based on photovoltaic cells further includes oxidizing the silicon wafer after low-temperature deposition after step (6); wherein the oxidation conditions are: nitrogen gas and oxygen are introduced at 80-300 Pascal and 700-730 degrees Celsius, and the oxidation time is 3-7 minutes; the oxygen flow rate is 800-3000 standard milliliters / minute, and the nitrogen gas flow rate is 800-3000 standard milliliters / minute.

[0062] This invention discovers that low-temperature annealing oxidation facilitates the incorporation of impurities into the silicon oxide layer, resulting in a better gettering effect.

[0063] In practice, those skilled in the art can remove the surface oxide layer and its metallic impurities by conventional processes such as wet processes after the impurities are incorporated into the silicon oxide layer through the above-mentioned low-temperature oxidation, thereby improving the surface passivation effect.

[0064] According to a preferred embodiment of the present invention, the heating rate of steps (2) and (3) is 12 to 16 degrees Celsius per minute.

[0065] According to a preferred embodiment of the present invention, the cooling rate of steps (5) and (6) is 3 to 5 degrees Celsius per minute.

[0066] According to a specific embodiment of the present invention, the variable-temperature diffusion process based on photovoltaic cells includes the following steps:

[0067] 1) The silicon wafers with the completed tunnel oxide layer and intrinsic amorphous silicon are loaded into a quartz boat and sent into a diffusion furnace;

[0068] 2) Raise the temperature of the diffusion furnace to 760-790 degrees Celsius and introduce nitrogen gas and oxygen for oxidation;

[0069] 3) Maintain the above temperature and introduce small nitrogen gas, oxygen gas and large nitrogen gas into the diffusion furnace to perform low-temperature deposition on the above-oxidized silicon wafer;

[0070] 4) Raise the temperature of the diffusion furnace by 30-40 degrees Celsius, shut off the oxygen and small nitrogen gas, and introduce large nitrogen gas for cryogenic propulsion;

[0071] 5) Raise the temperature of the diffusion furnace by 60-70 degrees Celsius and continue to introduce small nitrogen gas, oxygen and large nitrogen gas to perform high-temperature deposition on the silicon wafer after the above low-temperature deposition diffusion;

[0072] 6) Maintain the above temperature, shut off oxygen and small nitrogen, and introduce large nitrogen gas for propulsion.

[0073] 7) Reduce the temperature of the diffusion furnace by 20-50 degrees Celsius, and continue to introduce small nitrogen gas, oxygen and large nitrogen gas to perform medium-temperature deposition diffusion on the silicon wafer after the above high-temperature deposition diffusion.

[0074] 8) Reduce the temperature of the diffusion furnace by 80-100 degrees Celsius, and continue to introduce small nitrogen gas, oxygen and large nitrogen gas to perform low-temperature deposition diffusion on the silicon wafer after the above medium-temperature deposition diffusion.

[0075] 9) Reduce the temperature of the diffusion furnace to 700-730 degrees Celsius, shut off the small nitrogen gas supply, and introduce a large nitrogen gas supply and oxygen for oxidation;

[0076] 10) Break the vacuum and remove the silicon wafer after the above process is completed.

[0077] The relevant process parameters in step 2) are: oxygen flow rate of 300-1000 standard ml / min, nitrogen gas flow rate of 500-1500 standard ml / min, process pressure of 80-200 Pascal, and oxidation time of 1-5 minutes.

[0078] The relevant process parameters in step 3) are: small nitrogen gas flow rate of 800-1800 standard ml / min, oxygen flow rate of 300-800 standard ml / min, large nitrogen gas flow rate of 500-1000 standard ml / min, process pressure of 80-200 Pascal, and deposition time of 10-20 minutes.

[0079] The relevant process parameters in step 4) are: nitrogen gas flow rate of 1000-2000 standard ml / min, process pressure of 80-200 Pascal, and propagation time of 1-5 minutes.

[0080] The relevant process parameters in step 5) are as follows: small nitrogen gas flow rate is 800-1800 standard ml / min, oxygen flow rate is 300-800 standard ml / min, large nitrogen gas flow rate is 500-1000 standard ml / min, process pressure is 80-200 Pascal, and deposition time is 2-5 minutes.

[0081] The relevant process parameters in step 6) are: nitrogen gas flow rate of 1000-2000 standard ml / min, process pressure of 80-200 Pascal, and propagation time of 8-18 minutes.

[0082] The relevant process parameters in step 7) are as follows: small nitrogen gas flow rate is 800-1800 standard ml / min, oxygen flow rate is 300-1200 standard ml / min, large nitrogen gas flow rate is 400-1000 standard ml / min, process pressure is 80-200 Pascal, and deposition time is 2-6 minutes.

[0083] The relevant process parameters in step 8) are as follows: small nitrogen gas flow rate is 600-1800 standard ml / min, oxygen flow rate is 500-3000 standard ml / min, large nitrogen gas flow rate is 600-2000 standard ml / min, process pressure is 80-300 Pascal, and deposition time is 5-10 minutes.

[0084] The relevant process parameters in step 9) are: oxygen flow rate of 800-3000 standard ml / min, nitrogen gas flow rate of 800-3000 standard ml / min, process pressure of 80-300 Pascal, and deposition time of 3-7 minutes.

[0085] The present invention further provides a silicon wafer, which is prepared by the above-described variable temperature diffusion process.

[0086] The present invention also provides a photovoltaic cell comprising the aforementioned silicon wafer.

[0087] According to a preferred embodiment of the present invention, the photovoltaic cell is a TOPCon cell.

[0088] The present invention also provides the application of the above-mentioned silicon wafer in the field of photovoltaic cells.

[0089] Example 1

[0090] This embodiment provides a silicon wafer, the preparation method of which includes the following steps:

[0091] (1) The silicon wafers with the completed tunnel oxide layer and intrinsic amorphous silicon are loaded into a quartz boat and sent into a diffusion furnace.

[0092] (2) Raise the temperature of the diffusion furnace to 780 degrees Celsius, introduce 600 standard ml / min of oxygen and 1200 standard ml / min of nitrogen gas, maintain the process pressure at 120 Pascals, and oxidize for 3 minutes.

[0093] (3) Maintain the above temperature, introduce 1450 standard ml / min of small nitrogen, 600 standard ml / min of oxygen, and 800 standard ml / min of large nitrogen gas into the diffusion furnace, maintain the process pressure at 120 Pascal, and perform low-temperature deposition on the above oxidized silicon wafer for 15 minutes.

[0094] (4) Raise the temperature of the diffusion furnace to 820 degrees Celsius, shut off the oxygen and small nitrogen, introduce large nitrogen gas at 1600 standard ml / min, maintain the process pressure at 120 Pascals, and advance for 2 minutes;

[0095] (5) Raise the temperature of the diffusion furnace to 880 degrees Celsius, introduce 1450 standard milliliters / minute of small nitrogen, 600 standard milliliters / minute of oxygen, and 800 standard milliliters / minute of large nitrogen gas into the diffusion furnace, maintain the process pressure at 120 Pascals, and perform high-temperature deposition on the above-diffused silicon wafer for 3 minutes.

[0096] (6) Maintain the above temperature, turn off oxygen and small nitrogen, introduce large nitrogen gas at 1600 standard ml / min, maintain the process pressure at 120 Pascal, and advance for 15 minutes;

[0097] (7) Reduce the temperature of the diffusion furnace to 840 degrees Celsius, introduce 1000 standard ml / min of small nitrogen, 1000 standard ml / min of oxygen and 800 standard ml / min of large nitrogen into the diffusion furnace, maintain the process pressure at 120 Pascal, and perform medium-temperature deposition diffusion on the silicon wafer after the above high-temperature deposition diffusion for 5 minutes.

[0098] (8) Reduce the temperature of the diffusion furnace to 760 degrees Celsius, introduce 1000 standard ml / min of small nitrogen, 2500 standard ml / min of oxygen and 1200 standard ml / min of large nitrogen gas into the diffusion furnace, maintain the process pressure at 300 Pascal, and perform low-temperature deposition diffusion on the silicon wafer after the above medium-temperature deposition diffusion for 8 minutes.

[0099] (9) Reduce the temperature of the diffusion furnace to 730 degrees Celsius, turn off the small nitrogen gas, introduce 2400 standard ml / min of oxygen and 1000 standard ml / min of large nitrogen gas, maintain the process pressure at 300 Pascals, and oxidize for 5 minutes;

[0100] (10) Break the vacuum and remove the silicon wafer after the above process is completed.

[0101] Example 2

[0102] This embodiment provides a silicon wafer, the preparation method of which includes the following steps:

[0103] (1) The silicon wafers with the completed tunnel oxide layer and intrinsic amorphous silicon are loaded into a quartz boat and sent into a diffusion furnace.

[0104] (2) Raise the temperature of the diffusion furnace to 790 degrees Celsius, introduce 650 standard ml / min of oxygen and 1400 standard ml / min of nitrogen gas, maintain the process pressure at 120 Pascals, and oxidize for 2.5 minutes;

[0105] (3) Maintain the above temperature, introduce 1550 standard ml / min of small nitrogen, 650 standard ml / min of oxygen, and 1000 standard ml / min of large nitrogen gas into the diffusion furnace, maintain the process pressure at 120 Pascal, and perform low-temperature deposition on the above oxidized silicon wafer for 12 minutes.

[0106] (4) Raise the temperature of the diffusion furnace to 820 degrees Celsius, shut off the oxygen and small nitrogen, introduce large nitrogen gas at 1600 standard ml / min, maintain the process pressure at 120 Pascals, and advance for 2 minutes;

[0107] (5) Raise the temperature of the diffusion furnace to 890 degrees Celsius, introduce 1550 standard milliliters / minute of small nitrogen, 650 standard milliliters / minute of oxygen, and 1000 standard milliliters / minute of large nitrogen gas into the diffusion furnace, maintain the process pressure at 120 Pascals, and perform high-temperature deposition on the above-diffused silicon wafer for 3 minutes.

[0108] (6) Maintain the above temperature, turn off oxygen and small nitrogen, introduce large nitrogen gas at 1600 standard ml / min, maintain the process pressure at 120 Pascal, and advance for 15 minutes;

[0109] (7) Reduce the temperature of the diffusion furnace to 850 degrees Celsius, introduce 1000 standard ml / min of small nitrogen, 1000 standard ml / min of oxygen and 800 standard ml / min of large nitrogen into the diffusion furnace, maintain the process pressure at 120 Pascal, and perform medium-temperature deposition diffusion on the silicon wafer after the above high-temperature deposition diffusion for 5 minutes.

[0110] (8) Reduce the temperature of the diffusion furnace to 770 degrees Celsius, introduce 1000 standard ml / min of small nitrogen, 2500 standard ml / min of oxygen, and 1200 standard ml / min of large nitrogen gas into the diffusion furnace, maintain the process pressure at 300 Pascals, and perform low-temperature deposition diffusion on the silicon wafer after the above medium-temperature deposition diffusion for 8 minutes.

[0111] (9) Reduce the temperature of the diffusion furnace to 740 degrees Celsius, turn off the small nitrogen gas, introduce 2500 standard ml / min of oxygen and 1000 standard ml / min of large nitrogen gas, maintain the process pressure at 300 Pascals, and oxidize for 5 minutes;

[0112] (10) Break the vacuum and remove the silicon wafer after the above process is completed.

[0113] Example 3

[0114] This embodiment provides a silicon wafer, the preparation method of which includes the following steps:

[0115] (1) The silicon wafers with the completed tunnel oxide layer and intrinsic amorphous silicon are loaded into a quartz boat and sent into a diffusion furnace.

[0116] (2) Raise the temperature of the diffusion furnace to 770 degrees Celsius, introduce 600 standard ml / min of oxygen and 1200 standard ml / min of nitrogen gas, maintain the process pressure at 120 Pascals, and oxidize for 3 minutes;

[0117] (3) Maintain the above temperature, introduce 1450 standard ml / min of small nitrogen, 600 standard ml / min of oxygen, and 800 standard ml / min of large nitrogen gas into the diffusion furnace, maintain the process pressure at 120 Pascal, and perform low-temperature deposition on the above oxidized silicon wafer for 15 minutes.

[0118] (4) Raise the temperature of the diffusion furnace to 800 degrees Celsius, shut off the oxygen and small nitrogen, introduce large nitrogen gas at 1600 standard ml / min, maintain the process pressure at 120 Pascals, and advance for 2 minutes;

[0119] (5) Raise the temperature of the diffusion furnace to 870 degrees Celsius, introduce 1450 standard milliliters / minute of small nitrogen, 600 standard milliliters / minute of oxygen, and 800 standard milliliters / minute of large nitrogen gas into the diffusion furnace, maintain the process pressure at 120 Pascals, and perform high-temperature deposition on the above-diffused silicon wafer for 3 minutes.

[0120] (6) Maintain the above temperature, shut off oxygen and small nitrogen, introduce large nitrogen gas at 1600 standard ml / min, maintain the process pressure at 120 Pascal, and advance for 16 minutes;

[0121] (7) Reduce the temperature of the diffusion furnace to 830 degrees Celsius, introduce 1000 standard ml / min of small nitrogen, 1000 standard ml / min of oxygen and 800 standard ml / min of large nitrogen into the diffusion furnace, maintain the process pressure at 120 Pascals, and perform medium-temperature deposition diffusion on the silicon wafer after the above high-temperature deposition diffusion for 5 minutes.

[0122] (8) Reduce the temperature of the diffusion furnace to 750 degrees Celsius, introduce 1000 standard ml / min of small nitrogen, 2500 standard ml / min of oxygen, and 1200 standard ml / min of large nitrogen gas into the diffusion furnace, maintain the process pressure at 300 Pascals, and perform low-temperature deposition diffusion on the silicon wafer after the above medium-temperature deposition diffusion for 8 minutes.

[0123] (9) Reduce the temperature of the diffusion furnace to 720 degrees Celsius, turn off the small nitrogen gas, introduce 2400 standard ml / min of oxygen and 1000 standard ml / min of large nitrogen gas, maintain the process pressure at 300 Pascals, and oxidize for 5 minutes;

[0124] (10) Break the vacuum and remove the silicon wafer after the above process is completed.

[0125] Comparative Example 1

[0126] This comparative example provides a silicon wafer, which is prepared in the same way as in Example 1, except that steps (7) to (8) are not performed.

[0127] Comparative Example 2

[0128] This comparative example provides a silicon wafer, which is prepared in the same way as in Example 1, except that step (9) is not performed.

[0129] Test case

[0130] By processing the silicon wafers prepared in Examples 1-3 and Comparative Examples 1-2, and then fabricating them into batteries, the electrical performance data of the examples showed significant advantages over those of the silicon wafers prepared in Comparative Examples 1-2. Specifically, the open-circuit voltage of the battery containing the silicon wafer of Example 1 was increased by 0.17% compared to Comparative Example 1 and by 0.04% compared to Comparative Example 2; the short-circuit current was increased by 0.05% compared to Comparative Example 1 and by 0.02% compared to Comparative Example 2; the fill factor was increased by 0.22% compared to Comparative Example 1 and by 0.07% compared to Comparative Example 2; and the conversion efficiency was increased by 0.44% compared to Comparative Example 1 and by 0.13% compared to Comparative Example 2.

[0131] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A variable-temperature diffusion process based on photovoltaic cells, characterized in that, Includes the following steps: (1) Low-temperature deposition of silicon wafers with completed tunnel oxide layer and intrinsic amorphous silicon at 760-790 degrees Celsius; (2) Increase the temperature by 30-40 degrees Celsius to conduct low-temperature propulsion; (3) Increase the temperature by 60-70 degrees Celsius to perform high-temperature deposition; (4) Maintain the temperature of step (3) and carry out high-temperature propulsion; (5) Lower the temperature by 20-50 degrees Celsius and perform medium-temperature deposition; (6) Lower the temperature by 80-100 degrees Celsius and perform low-temperature deposition; Steps (1), (3), (5), and (6) are performed under conditions of introducing small amounts of nitrogen gas, oxygen gas, and large amounts of nitrogen gas; steps (2) and (4) are performed under conditions of introducing only large amounts of nitrogen gas. In steps (1) and (3), the flow rate of small nitrogen gas is 800-1800 standard milliliters / minute, the flow rate of oxygen is 300-800 standard milliliters / minute, and the flow rate of large nitrogen gas is 500-1000 standard milliliters / minute; In steps (2) and (4), the flow rate of nitrogen gas is 1000-2000 standard milliliters / minute; In step (5), the flow rate of small nitrogen gas is 800-1800 standard milliliters / minute, the flow rate of oxygen is 300-1200 standard milliliters / minute, and the flow rate of large nitrogen gas is 400-1000 standard milliliters / minute; In step (6), the flow rate of small nitrogen gas is 600-1800 standard milliliters / minute, the flow rate of oxygen is 500-3000 standard milliliters / minute, and the flow rate of large nitrogen gas is 600-2000 standard milliliters / minute.

2. The variable-temperature diffusion process based on photovoltaic cells according to claim 1, characterized in that, In step (1), the low-temperature deposition time is 10 to 20 minutes; In step (2), the cryogenic propulsion time is 1 to 5 minutes; In step (3), the high-temperature deposition time is 2 to 5 minutes; In step (4), the high-temperature propulsion time is 8 to 18 minutes; In step (5), the medium-temperature deposition time is 2 to 6 minutes; In step (6), the low-temperature deposition time is 5 to 10 minutes.

3. The variable-temperature diffusion process based on photovoltaic cells according to claim 1, characterized in that, Also includes: Before step (1), the silicon wafer with the completed tunnel oxide layer and intrinsic amorphous silicon is oxidized; wherein the oxidation conditions are: nitrogen gas and oxygen are introduced at 80-200 Pascal and 760-790 degrees Celsius, and the oxidation time is 1-5 minutes; the oxygen flow rate is 300-1000 standard milliliters / minute, and the nitrogen gas flow rate is 500-1500 standard milliliters / minute.

4. The variable-temperature diffusion process based on photovoltaic cells according to any one of claims 1 to 3, characterized in that, Also includes: After step (6), the silicon wafer deposited at low temperature is oxidized; wherein the oxidation conditions are: nitrogen gas and oxygen are introduced at 80-300 Pascal and 700-730 degrees Celsius, and the oxidation time is 3-7 minutes; the oxygen flow rate is 800-3000 standard milliliters / minute, and the nitrogen gas flow rate is 800-3000 standard milliliters / minute.

5. The variable-temperature diffusion process based on photovoltaic cells according to any one of claims 1 to 3, characterized in that, The heating rate in steps (2) and (3) is 12 to 16 degrees Celsius per minute; The cooling rate in steps (5) and (6) is 3 to 5 degrees Celsius per minute.

6. A silicon wafer, characterized in that, It is prepared by any one of the variable temperature diffusion processes according to claims 1 to 5.

7. A photovoltaic cell, characterized in that, Contains the silicon wafer as described in claim 6.

8. The photovoltaic cell according to claim 7, characterized in that, The photovoltaic cell is a TOPCon cell.

9. The application of the silicon wafer according to claim 6 in the field of photovoltaic cells.

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