Preparation method of solar cell

Abstract

The invention discloses a preparation method of a solar cell, and the method comprises the following steps: placing an N-type silicon wafer in a furnace tube of a diffusion furnace, and introducing nitrogen and oxygen into the furnace tube to form a first oxide layer on the surface of the silicon wafer; boron trichloride, oxygen and nitrogen are introduced into the furnace tube, so that a boron diffusion layer is formed on the surface, away from the silicon wafer, of the first oxide layer; introducing nitrogen into the furnace tube, and controlling the temperature of the furnace tube to be a first temperature so as to carry out oxygen-free propulsion of the boron element; introducing nitrogen and oxygen into the furnace tube, and controlling the temperature of the furnace tube to rise from the first temperature to a second temperature so as to carry out aerobic propulsion of the boron element; dry oxygen and wet oxygen are alternately introduced into the furnace tube for multiple times to form a second oxide layer; the temperature in the furnace tube is reduced, and dry oxygen continues to be introduced into the furnace tube; and nitrogen is introduced into the furnace tube, the temperature in the furnace tube is reduced, and boron diffusion of the silicon wafer is completed. According to the preparation method of the solar cell, the silicon wafer can have low surface concentration shallow junctions and uniform sheet resistance.

Description

Technical Field

[0001] The invention relates to the technical field of solar cells, and in particular to a method for preparing a solar cell. Background Art

[0002] Passivated contact solar cells made of N-type silicon wafers have the advantages of high efficiency, long life, no LID (Light Induced Degradation), good weak light response, high limit efficiency, etc., and have attracted much attention and in-depth research from domestic and foreign manufacturers and researchers. When preparing solar cells, boron elements need to be diffused into N-type silicon wafers to form PN junctions.

[0003] In the prior art, an N-type silicon wafer is placed in a diffusion furnace and a boron diffusion layer is formed on its surface, an oxide layer is formed on the surface of the silicon wafer, and oxygen is introduced for aerobic propulsion, so that the boron element in the boron diffusion layer diffuses into the silicon substrate, so that the silicon substrate can be doped with boron elements to form a PN junction. However, the distribution uniformity of the boron element in the silicon substrate is poor. Summary of the invention

[0004] Based on the above situation, in order to overcome the shortcomings of the prior art, the present invention provides a method for preparing a solar cell with strong operability, simple process conditions, low cost, effectiveness and uniform square resistance. The present invention provides the following technical solutions:

[0005] A method for preparing a solar cell comprises: S1. placing an N-type silicon wafer in a furnace tube of a diffusion furnace, and introducing nitrogen and oxygen into the furnace tube to form a first oxide layer on the surface of the silicon wafer; S2. introducing boron trichloride, oxygen and nitrogen into the furnace tube to form a boron diffusion layer on the surface of the first oxide layer away from the silicon wafer; S3. introducing nitrogen into the furnace tube and controlling the temperature of the furnace tube to be a first temperature to carry out oxygen-free propulsion of the boron element; S4. introducing nitrogen and oxygen into the furnace tube and controlling the temperature of the furnace tube to rise from the first temperature to a second temperature to carry out oxygen-containing propulsion of the boron element; S5. alternately introducing dry oxygen and wet oxygen into the furnace tube for multiple times to form a second oxide layer; S6. reducing the temperature in the furnace tube and continuing to introduce the dry oxygen into the furnace tube; S7. introducing nitrogen into the furnace tube and reducing the temperature in the furnace tube to complete the boron diffusion of the silicon wafer.

[0006] Optionally, the step S5 includes: continuously introducing nitrogen and oxygen into the furnace tube to form a dry oxygen atmosphere, and intermittently introducing nitrogen purified by pure water into the furnace tube to form a wet oxygen atmosphere.

[0007] Optionally, in the dry oxygen atmosphere, the flow rate of the nitrogen gas is 600 sccm to 1300 sccm, and the flow rate of the oxygen gas is 20000 sccm to 30000 sccm; in the wet oxygen atmosphere, the flow rate of the nitrogen gas is 1000 sccm to 1500 sccm.

[0008] Optionally, in step S5, the temperature of the furnace tube is controlled to be 950° C. to 1040° C., the pressure is controlled to be 600 mbar to 800 mbar, and the time is controlled to be 30 min to 50 min.

[0009] Optionally, the step S5 includes: alternately introducing a first dry oxygen, a first wet oxygen, a second dry oxygen, a second wet oxygen and a third dry oxygen into the furnace tube multiple times in sequence.

[0010] Optionally, in step S1, the temperature in the furnace tube is controlled to be 790° C. to 870° C., the flow rate of nitrogen is 500 sccm to 4000 sccm, the flow rate of oxygen is 3000 sccm to 6000 sccm, and the processing time is 5 min to 8 min.

[0011] Optionally, in step S2, the temperature in the furnace tube is controlled to be 800° C. to 880° C., the pressure is controlled to be 160 mbar to 210 mbar, the flow rate of boron trichloride is controlled to be 350 sccm to 600 sccm, and the flow rate of nitrogen is controlled to be 500 sccm to 4000 sccm.

[0012] Optionally, in step S3, the first temperature is 860°C to 900°C, the time for anaerobic advancement is 2min to 8min, and the flow rate of nitrogen is 500sccm to 4000sccm; and / or, in step S4, the second temperature is 950°C to 1040°C, the flow rate of oxygen is 10000sccm to 25000sccm, and the time for aerobic advancement is 20min to 30min.

[0013] Optionally, in step S6, the temperature in the furnace tube is controlled to be 930° C. to 980° C., and the flow rate of the dry oxygen is 10000 sccm to 30000 sccm.

[0014] Optionally, before step S1, the method further includes: introducing 3500-4500 sccm of nitrogen into the furnace tube to make the air pressure in the furnace tube equal to the external air pressure, and controlling the temperature in the furnace tube to be 750°C-780°C; reducing the pressure in the furnace tube to 600mbar-700mbar, and controlling the furnace tube to heat up to 790°C-825°C within 10min-12min, reducing the pressure to 160mbar-210mbar, and maintaining the temperature for 2min-5min; after the pressure in the furnace tube is pumped down to 160mbar-210mbar, closing the air inlet valve and the air exhaust valve, and performing vacuum leak detection on the furnace tube; and / or, after step S7, the method further includes: removing the second oxide layer, the boron diffusion layer, and the first oxide layer from at least a portion of the surface of the silicon wafer.

[0015] The method for preparing a solar cell provided by an embodiment of the present invention forms a first oxide layer on the surface of a silicon wafer, and forms a boron diffusion layer on the surface of the first oxide layer away from the silicon wafer, then performs aerobic advancement at a first temperature and aerobic advancement at a second temperature higher than the first temperature, and then alternately introduces dry oxygen and wet oxygen into the furnace tube multiple times to form a second oxide layer, and the compactness of the second oxide layer is better than the compactness of the second oxide layer formed by all wet oxygen, and the preparation efficiency is higher than that of the second oxide layer formed by all dry oxygen. In this way, through the coordination of the first oxide layer, the anaerobic advancement at the first temperature, the aerobic advancement at the second temperature and the second oxide layer, the silicon wafer can have a low surface concentration shallow junction and uniform square resistance. And the prepared solar cell has low surface recombination, which is conducive to improving the open circuit voltage and short circuit current. In addition, the second oxide layer is prepared by alternately introducing dry oxygen and wet oxygen multiple times, which is conducive to extending the service life of furnace tubes, carriers and other related equipment relative to the formation of the second oxide layer entirely by dry oxygen. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For purposes of illustration and not limitation, the present invention will now be described in terms of preferred embodiments thereof, with particular reference to the accompanying drawings, in which:

[0017] Figure 1 A schematic diagram of a process for preparing a solar cell according to an embodiment of the present invention;

[0018] FIG. 2A to FIG. 2F They are respectively the comparison diagrams of the square resistance test of Example 1 of the present invention and the comparative example;

[0019] Figure 3 This is a comparison curve of the ECV test of Example 1 of the present invention and the comparative example. The horizontal axis represents the junction depth (in μm), and the vertical axis represents the surface concentration (n / cm 3 ). DETAILED DESCRIPTION

[0020] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] Diffusion is the core of preparing the PN junction of solar cells. Diffusion is divided into constant surface source diffusion and limited surface source diffusion. Under ideal conditions, the constant source keeps the surface concentration constant and has nothing to do with the diffusion time; while limited source diffusion is that the total impurity concentration remains unchanged over time while the substrate surface concentration decreases while the diffusion temperature remains constant. At present, it is easier to ensure the uniformity of the N-type silicon wafer substrate surface by thermal equilibrium high concentration diffusion, but it is difficult to control the formation of a boron-rich layer on the surface at high concentration; on the contrary, low concentration diffusion can avoid this problem to a certain extent, but the uniformity is poor. At the same time, the process of TOPCon cells (Tunnel Oxide Passivated Contact solar cells) includes high-temperature boron diffusion, formation of selective emitters, and oxidation of secondary boron diffusion to ensure the quality of the PN junction prepared on the front and better control the doping concentration, but the process is long and the cost is high. Traditional thermal oxidation dry oxygen takes a long time and high temperature, and the film growth quality is high, but the growth rate is very low; and wet oxygen oxidation is still an inevitable problem of sparse oxide layer and low film quality due to the fast growth rate. In this way, it is not easy to prepare a solar cell with high junction quality, good silicon wafer square resistance or diffusion concentration uniformity.

[0022] Based on the above, an embodiment of the present invention provides a method for preparing a solar cell, such as Figure 1 As shown, the method may mainly include the following steps S1 to S7.

[0023] S1. Place an N-type silicon wafer in a furnace tube of a diffusion furnace, and introduce nitrogen and oxygen into the furnace tube to form a first oxide layer on the surface of the silicon wafer.

[0024] S2. Boron trichloride, oxygen and nitrogen are introduced into the furnace tube to form a boron diffusion layer on the surface of the first oxide layer away from the silicon wafer.

[0025] S3. Introduce nitrogen into the furnace tube and control the temperature of the furnace tube to be a first temperature to perform oxygen-free propulsion of the boron element.

[0026] S4. Nitrogen and oxygen are introduced into the furnace tube, and the temperature of the furnace tube is controlled to rise from a first temperature to a second temperature to carry out aerobic propulsion of the boron element.

[0027] S5. Dry oxygen and wet oxygen are introduced alternately into the furnace tube multiple times to form a second oxide layer.

[0028] S6. Lower the temperature inside the furnace tube and continue to introduce dry oxygen into the furnace tube.

[0029] S7. Introduce nitrogen into the furnace tube and lower the temperature inside the furnace tube to complete the boron diffusion of the silicon wafer.

[0030] The method for preparing a solar cell provided by an embodiment of the present invention forms a first oxide layer on the surface of a silicon wafer, and forms a boron diffusion layer on the surface of the first oxide layer away from the silicon wafer, then performs aerobic advancement at a first temperature and aerobic advancement at a second temperature higher than the first temperature, and then alternately introduces dry oxygen and wet oxygen into the furnace tube multiple times to form a second oxide layer. The compactness of the second oxide layer is better than the compactness of the second oxide layer formed by all wet oxygen, and the efficiency of forming the second oxide layer is higher than that of forming the second oxide layer by all dry oxygen. In this way, through the coordination of the first oxide layer, the anaerobic advancement at the first temperature, the aerobic advancement at the second temperature and the second oxide layer, the silicon wafer can have a low surface concentration shallow junction and uniform square resistance, and the prepared solar cell has low surface recombination, which is beneficial to improve the open circuit voltage and short circuit current. In addition, the second oxide layer is prepared by alternately introducing dry oxygen and wet oxygen multiple times, which is beneficial to extend the service life of furnace tubes, carriers (such as quartz boats) and other related equipment relative to the formation of the second oxide layer entirely by dry oxygen.

[0031] The first oxide layer is provided to facilitate the dispersion of the boron element when it diffuses into the silicon wafer. Specifically, the oxygen-free advancement at the first temperature allows more boron atoms to enter the first oxide layer from the boron diffusion layer and approach the silicon wafer. At this time, the boron element will be dispersed in the first oxide layer, avoiding the accumulation of high-concentration doped atoms on the surface, thereby reducing defects and recombination centers. The oxygen advancement at the second temperature higher than the first temperature can better deposit the boron element on the surface of the silicon wafer, so that a large amount of boron accumulates on the surface of the silicon wafer to form a high-concentration area, which is conducive to the uniform diffusion of boron.

[0032] Furthermore, by using the variable temperature advancement of the first temperature oxygen-free advancement and the second temperature oxygen advancement, the efficiency can be improved and the uniformity of the square resistance in the silicon wafer can be improved under the same boron diffusion square resistance condition. In addition, the variable temperature advancement can be carried out in a relatively low temperature environment, which reduces the damage rate of the velvet pyramid formed on the silicon wafer, and is conducive to maintaining the integrity of the silicon wafer surface structure.

[0033] Then, based on the different segregation coefficients and solid solubility differences of boron in the silicon oxide layer and silicon, a second oxide layer is formed to further regulate the junction depth and diffusion concentration of the boron element on the silicon wafer surface.

[0034] Through the coordination of the first oxide layer and the second oxide layer, the diffusion rate of the doping elements can be regulated, the uniformity of the oxide layer and the pn junction can be improved, and the surface recombination and diffusion dead layer can be reduced, thereby improving the concentration shallow junction and uniform square resistance and enhancing the performance of solar cells.

[0035] In addition, the formation of a second oxide layer can affect the diffusion process by controlling the diffusion rate, improving doping uniformity and improving the quality of the pn junction. Moreover, the second oxide layer has good density and can improve the fill factor by reducing surface recombination, improving carrier transport, enhancing electric field passivation, reducing contact resistance and improving battery stability.

[0036] In the embodiment of the present invention, for the convenience of description, the precursor of the solar cell obtained in step S1 to step S7 may also be referred to as a processed silicon wafer, and for a simpler description, may also be referred to as a silicon wafer.

[0037] In order to more clearly understand the method for preparing the solar cell provided by the present invention, a more detailed description is given below.

[0038] Before step S1, the following loading preparation operation can also be performed: put the N-type substrate silicon wafer that has been texturized and cleaned into a boat and then place it in a diffusion furnace, and introduce nitrogen at 3500sccm to 4500sccm to make the air pressure in the furnace tube equal to the external air pressure, such as 3500sccm, 3600sccm, 3700sccm, 3800sccm, 3900sccm, 4000sccm, 4200sccm or 4500sccm; and control the temperature in the furnace tube to 750℃~780℃, such as 750℃, 760℃, 770℃ or 780℃. Subsequently, the pressure in the furnace tube is reduced to 600mbar-700mbar, such as 600mbar, 625mbar, 650mbar, 675mbar or 700mbar, etc.; at the same time, the furnace tube is controlled to heat up to 790℃-825℃, such as 790℃, 795℃, 800℃, 810℃, 820℃ or 825℃, etc. within 10min-12min. The pressure is reduced to 160mbar-210mbar, such as 160mbar, 185mbar or 210mbar, etc.; the temperature is kept constant for 2min-5min, such as 2min, 3min, 4min or 5min, etc.; after the pressure in the furnace tube is pumped to 160mbar-210mbar, such as 160mbar, 185mbar or 210mbar, etc., the air inlet valve and the air extraction valve are closed, and the furnace tube is vacuum leak tested.

[0039] In step S1, the purpose is to generate a first oxide layer under a dual atmosphere of nitrogen and oxygen. The specific details are: the processing temperature is 790°C to 870°C, such as 790°C, 810°C, 830°C, 850°C or 870°C; the nitrogen flow rate is 500sccm to 4000sccm, such as 500sccm, 1000sccm, 2000sccm, 3000sccm, 3500sccm or 3500sccm, etc.; the oxygen flow rate is 3000sccm to 6000sccm, such as 3000sccm, 4000sccm, 5000sccm or 6000sccm, etc.; the processing time is 5min to 8min, such as 5min, 6min, 7min or 8min, etc.

[0040] In step S2 , the purpose is to form a boron diffusion layer on the surface of the silicon substrate by through-source deposition. The specific details are as follows: the flow ratio of boron trichloride to oxygen is about 1:3, the deposition temperature is 800°C to 880°C (lower than the deposition temperature of 870°C to 950°C in the prior art), for example, it can be 800°C, 820°C, 840°C, 860°C or 880°C; the deposition pressure is 160mbar to 210mbar, for example, it can be 160mbar, 170mbar, 185mbar, 200mbar or 210mbar, etc.; the boron trichloride flow rate is 350sccm to 600sccm, for example, it can be 350sccm, 400sccm, 450sccm, 500sccm, 550sccm or 600sccm, etc.; the nitrogen flow rate is 500sccm to 4000sccm, for example, it can be 500sccm, 1000sccm, 2000sccm, 3000sccm, 3500sccm or 4000sccm, etc.

[0041] In step S3, the purpose is to promote boron diffusion for the first time under anaerobic conditions. The specific details are: the first temperature is 860℃~900℃, for example, it can be 860℃, 870℃, 880℃, 890℃ or 900℃; the time for anaerobic promotion is 2min~8min, for example, it can be 5min, 6min, 7min or 8min, etc.

[0042] In step S4, the purpose is to promote boron diffusion for the second time under aerobic conditions. The specific details are: the second temperature is 950℃~1040℃, for example, it can be 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, 1010℃, 1020℃, 1030℃ or 1040℃; the oxygen flow rate is 10000sccm~25000sccm, for example, it can be 10000sccm, 15000sccm, 20000sccm or 25000sccm, etc.; the aerobic promotion time is 20min~30min, for example, it can be 20min, 25min or 30min, etc.

[0043] In step S5, the purpose is to perform laminate oxidation treatment in an alternating atmosphere of dry oxygen and wet oxygen. The specific details are: nitrogen and oxygen are continuously introduced into the furnace tube to form a dry oxygen atmosphere, and nitrogen that has been purified by water is intermittently introduced into the furnace tube to form a wet oxygen atmosphere. In a dry oxygen atmosphere, the flow rate of nitrogen is 600sccm~1300sccm, for example, it can be 600sccm, 700sccm, 800sccm, 900sccm, 1000sccm, 1100sccm, 1200sccm or 1300sccm, etc.; the flow rate of oxygen is 20000sccm~30000sccm, for example, it can be 20000sccm, 22500sccm, 25000sccm, 27500sccm or 30000sccm, etc.; in a wet oxygen atmosphere, the flow rate of nitrogen is 1000sccm~1500sccm, for example, it can be 1000sccm, 1100sccm, 1200sccm, 1300sccm, 1400sccm or 1500sccm, etc. In this step, the temperature of the furnace tube is controlled to be 950°C to 1040°C, such as 950°C, 970°C, 980°C, 1000°C, 1020°C or 1040°C; the pressure is 600mbar to 800mbar, such as 600mbar, 650mbar, 700mbar, 750mbar or 800mbar, etc.; the processing time is 30min to 50min, such as 30min, 35min, 40min, 45min or 50min, etc. Preferably, the dry oxygen and wet oxygen are alternately arranged multiple times and sequentially include: the first dry oxygen, the first wet oxygen, the second dry oxygen, the second wet oxygen and the third dry oxygen. The thickness and density of the oxide layer can be controlled by controlling the processing time and gas flow rate of each link (i.e., the five links of three dry oxygen and two wet oxygen).

[0044] In step S6, the purpose is annealing crystallization treatment, and the specific details are: controlling the temperature in the furnace tube to 930°C ~ 980°C, for example, it can be 930°C, 940°C, 950°C, 960°C, 970°C or 980°C; the oxygen flow rate is 10000sccm ~ 30000sccm, for example, it can be 10000sccm, 15000sccm, 20000sccm, 25000sccm or 30000sccm, etc.

[0045] The purpose of step S7 is to unload the product. The specific details are as follows: nitrogen is introduced to break the vacuum until the pressure is equal to the external atmospheric pressure. When the diffusion furnace is cooled to room temperature, the furnace door is opened to take out the quartz boat, and the processed N-type silicon wafer is taken out from the quartz boat.

[0046] After step S7, the following step may be further included: removing the second oxide layer, the boron diffusion layer, and the first oxide layer from at least a portion of the surface of the silicon wafer, so that at least a portion of the silicon wafer can be exposed to facilitate the arrangement of other film layers on the surface.

[0047] In view of the defects of the existing production line processing methods such as high requirements for silicon wafer boron diffusion junction temperature, uneven square resistance, high surface concentration, and short equipment life, the preparation method of the solar cell of the embodiment of the present invention first deposits a boron diffusion layer on the surface of the silicon wafer at a low temperature, and then performs temperature gradient anaerobic and aerobic advancement on the silicon wafer deposited with the boron diffusion layer. Considering that the diffusion rate of boron atoms is affected by the diffusion temperature and the concentration of boron atoms, the diffusion rate will increase with the increase of the diffusion temperature at different boron atom concentrations, so variable temperature is conducive to diffusion. After the low-temperature aerobic advancement step, water vapor is added to form a dry oxygen-wet oxygen-dry oxygen-wet oxygen-dry oxygen laminated structure oxide layer, which not only overcomes the problem of too fast growth and poor density of the water vapor oxide layer, and ensures the density of the oxide layer, but also has a certain protective effect on the furnace tube (usually around 900°C or 1000°C, the use of wet oxygen can play a cooling role, thereby protecting the furnace tube). In addition, the temperature of the aerobic advancement step is higher than that of the anaerobic advancement. In addition to effectively improving the uniformity of boron diffusion resistance through two temperature-changing advancements, the entire diffusion step eliminates the boron source replenishment to form a low surface concentration shallow junction, greatly reducing the surface series resistance of the silicon wafer and thus reducing recombination, thereby improving the overall performance of the solar cell.

[0048] Specifically, the boron source deposited at low temperature (800°C to 880°C in the present invention) can push boron doping into the silicon wafer during the oxygen-free advancement process, and the boron doping concentration at the edge of the silicon wafer is higher than that at the center of the silicon wafer. On the basis of canceling the boron source supplementation, aerobic advancement is continued, and dry oxygen-wet oxygen-dry oxygen-wet oxygen-dry oxygen is cyclically introduced under an oxygen atmosphere. The thickness of the oxide layer formed at the edge of the silicon wafer is greater than that at the center of the silicon wafer. The difference in the partial condensation coefficient and solid solubility of boron in silicon dioxide and silicon can be used to regulate the surface concentration and junction depth of the silicon wafer, so that the silicon wafer takes into account uniformity while ensuring the junction depth quality and surface concentration, which is conducive to improving the performance of solar cells. More importantly, the boron diffusion layer deposition and oxygen-free advancement under relatively low temperature conditions can reduce the temperature difference between the edge and the center during the heating process, and control the doping amount of boron entering the silicon substrate to ensure doping uniformity. Continuing aerobic advancement makes it easier to use the different solid solubility of boron in silicon and silicon oxide to achieve effective regulation of surface concentration and junction depth, and thus does not require laser selective doping and secondary oxidation of boron diffusion. In particular, the elimination of selective doping can effectively reduce the laser damage layer on the surface of the silicon wafer, and the excellent PN junction depth and surface concentration can obtain a solar cell with excellent electrical performance. In addition, the present invention also has the advantages of simple process, convenient operation, low cost, etc., which is conducive to large-scale production and wide application, with high use value and good application prospects.

[0049] In order to enable those skilled in the art to better understand, specific examples are listed below for illustration.

[0050] Example 1

[0051] Example 1 adopts the method for preparing a solar cell of the present invention, and the specific process is as follows.

[0052] (1) Place the N-type substrate silicon wafer that has been texturized and cleaned into a quartz boat and then place it in a diffusion furnace. Pass 4000sccm of nitrogen until it is equal to the local atmospheric pressure, and the temperature is raised to 780°C. The diffusion furnace is evacuated to 700mbar and the temperature is raised to 820°C over 10 minutes. The temperature of the diffusion furnace is maintained constant for 3 minutes. The furnace tube is depressurized to 160mbar, the air inlet / exhaust valve is closed, and the diffusion furnace is vacuumed for leak detection.

[0053] (2) The furnace temperature was set to 820° C., the nitrogen flow rate was 500 sccm, the oxygen flow rate was 4000 sccm, and the processing time was 7 min to form a first oxide layer.

[0054] (3) 350 sccm of boron trichloride, 1050 sccm of oxygen, and 500 sccm of nitrogen are introduced at a deposition temperature of 820° C. and a deposition pressure of 200 mbar to form a boron diffusion layer on the surface of the first oxide layer facing away from the silicon wafer.

[0055] (4) The temperature is raised to a first temperature of 860° C., 4000 sccm of nitrogen is introduced, and the oxygen-free advancement of the boron element is performed for 4 minutes.

[0056] (5) The temperature was raised to a second temperature of 1000° C., nitrogen was introduced, and at the same time, 10000 sccm of oxygen was introduced to carry out aerobic propulsion of the boron element for 25 minutes.

[0057] (6) The furnace temperature is set to 1000°C and the pressure is set to 600 mbar. The treatment can be carried out in the order of "dry oxygen-wet oxygen-dry oxygen-wet oxygen-dry oxygen" to form a second oxide layer. In the dry oxygen treatment stage, 800 sccm of nitrogen and 20,000 sccm of oxygen are introduced at the same time. In the wet oxygen treatment stage, nitrogen passed through a pure water bottle is introduced, and the amount of nitrogen introduced is 1000 sccm.

[0058] (7) Cooling to the annealing temperature of 940°C. During the cooling period, the oxygen gas flow rate was maintained at 10,000 sccm, and the nitrogen gas flow through the pure water bottle and the nitrogen purge were stopped.

[0059] (8) Introduce nitrogen, cool to room temperature, break the vacuum and return the pressure, take out the quartz boat, and take out the silicon wafer.

[0060] Example 2

[0061] Example 2 is prepared using the method for preparing a solar cell of the present invention, and the specific process is as follows.

[0062] (1) Place the N-type substrate silicon wafer that has been texturized and cleaned into a quartz boat and then place it in a diffusion furnace. Pass 4000 sccm of nitrogen until it is equal to the local atmospheric pressure, and raise the temperature to 780°C. Evacuate the diffusion furnace to 600 mbar and raise the temperature to 810°C in 10 minutes. Keep the furnace temperature constant for 5 minutes. Evacuate the furnace tube to 180 mbar, close the air inlet / exhaust valve, and perform a vacuum leak test on the diffusion furnace.

[0063] (2) The furnace temperature was set to 810° C., the nitrogen flow rate was set to 600 sccm, the oxygen flow rate was set to 5000 sccm, and the processing time was set to 5 min to form a first oxide layer.

[0064] (3) Boron trichloride (400 sccm), oxygen (1200 sccm), and nitrogen (600 sccm) are introduced at a deposition temperature of 880° C. and a deposition pressure of 160 mbar to form a boron diffusion layer on the surface of the first oxide layer facing away from the silicon wafer.

[0065] (4) The temperature is raised to a first temperature of 900° C., 4000 sccm of nitrogen is introduced, and the boron element is oxygen-free promoted for 3 minutes.

[0066] (5) The temperature is raised to a second temperature of 950° C., nitrogen is introduced, and at the same time, 15,000 sccm of oxygen is introduced to carry out aerobic propulsion of the boron element for 20 minutes.

[0067] (6) The furnace temperature is set to 950°C and the pressure is set to 700 mbar. The treatment can be carried out in the order of "dry oxygen-wet oxygen-dry oxygen-wet oxygen-dry oxygen" to form a second oxide layer. In the dry oxygen treatment stage, 800 sccm of nitrogen and 30,000 sccm of oxygen are introduced at the same time. In the wet oxygen treatment stage, 1500 sccm of nitrogen passed through a pure water bottle is introduced.

[0068] (7) Cooling to the annealing temperature of 940°C. During the cooling period, the oxygen gas flow rate was maintained at 10,000 sccm, and the nitrogen gas flow through the pure water bottle and the nitrogen purge were stopped.

[0069] (8) Introduce nitrogen, cool to room temperature, break the vacuum and return the pressure, take out the quartz boat, and take out the silicon wafer.

[0070] Comparative Example

[0071] The comparative example adopts the existing method for preparing solar cells, which includes the following steps:

[0072] (1) Place the N-type substrate silicon wafer that has been texturized and cleaned into a quartz boat and then place it in a diffusion furnace, pass nitrogen, increase the furnace temperature, evacuate, and test the airtightness.

[0073] (2) Under the conditions of temperature of 860° C., pressure of 180 kPa, introduction of 300 sccm nitrogen and 5000 sccm oxygen, i.e., under dual atmosphere conditions, the reaction was carried out for 8 minutes to form a first oxide layer.

[0074] (3) Under the conditions of a temperature of 920° C., a pressure of 180 kPa, and the introduction of 7000 sccm of nitrogen, 540 sccm of oxygen, and 190 sccm of boron trichloride, i.e., under three-atmosphere conditions, the reaction was carried out for 20 minutes to form a boron diffusion layer.

[0075] (4) Deep diffusion was performed for 7 min at a temperature of 910°C and a pressure of 180 kPa with 7000 sccm of nitrogen gas introduced. Then, 7000 sccm of nitrogen gas, 700 sccm of oxygen gas and 210 sccm of boron trichloride gas were introduced under three-atmosphere conditions for source replenishment.

[0076] (5) Annealing was performed at 850°C with 20,000 sccm of nitrogen for 10 min, and then the vacuum was broken and the pressure was restored. The quartz boat and the silicon wafer were taken out.

[0077] (6) After the selective emitter treatment, the tube is reinstalled into the furnace and a second oxide layer is formed under the conditions of a temperature of 1050°C, a pressure of 700 kPa, and an oxygen flow of 25,000 sccm.

[0078] (7) Annealing treatment was performed at a temperature of 940°C, a pressure of 700 kPa, and a flow of 25,000 sccm of nitrogen. Finally, the vacuum was broken and the pressure was returned, and the quartz boat and the silicon wafer were taken out.

[0079] The silicon wafers were treated by the methods provided in Example 1, Example 2 and the comparative example, and then the treated silicon wafers were taken out at the furnace mouth, the furnace and the furnace tail, and the square resistance test was performed. As shown in Table 1, the square resistance test comparison of the silicon wafers obtained in Example 1, Example 2 and the comparative example is shown. As can be seen from the table, compared with the comparative example of the prior art, the square resistance of the silicon wafers located at the furnace mouth, the furnace and the furnace tail of Example 1 of the present invention is smaller, and the uniformity is better.

[0080] Table 1 Comparison of silicon wafer square resistance test between Example 1 and Comparative Example

[0081] Square resistance(ohm) Furnace mouth In the furnace Furnace tail Mean Example 1 312.7 314.6 315 314.1 Example 2 322.1 327.8 329.4 326.4 Comparative Example 303.3 266.7 260.1 276.7

[0082] FIG. 2A to FIG. 2C They are respectively the square resistance test contour diagrams of silicon wafers prepared by the method provided in Example 1 of the present invention and taken from the furnace mouth, the furnace and the furnace tail, respectively, FIG. 2D to FIG. 2F The square resistance test contour diagrams of silicon wafers taken from the furnace mouth, furnace middle and furnace tail are obtained by the method provided in the comparative example. FIG. 2A to FIG. 2C The square resistance at different positions of the same silicon wafer is characterized respectively. In a single picture, small color difference at different positions (such as blue and green) means that the square resistance values ​​at different positions are close and the uniformity is good. Large color difference at different positions (such as red and blue) means that the square resistance values ​​at different positions are very different and the uniformity is poor. By comparison, it can be seen that the intra-wafer uniformity (i.e. FIG. 2A to FIG. 2C The intra-sheet uniformity of the comparison ratio (i.e. FIG. 2D to FIG. 2F In addition, the inter-sheet uniformity of Example 1 (i.e. FIG. 2A to FIG. 2C Comparison of different pictures with each other) to compare the uniformity of the slices (i.e. FIG. 2D to FIG. 2F It is better to compare different pictures with each other).

[0083] like Figure 3The figure shows the ECV (Electrochemical capacitance-voltage profiler) test comparison of Example 1 and the prior art comparative example. The horizontal axis represents the junction depth (i.e., the diffusion junction depth); the vertical axis represents the surface concentration (i.e., the surface atomic concentration). The maximum doping concentration (surface concentration) of the silicon wafer obtained in Example 1 is 5.59E+18cm -3 The junction depth is 1.057 μm, and the maximum doping concentration (surface concentration) of the silicon wafer obtained in the comparative example is 7.61E+18 cm -3 , the junction depth is 1.226 μm. It can be seen that the doping surface concentration of the silicon wafer obtained in Example 1 is lower and the junction depth is also shallower.

[0084] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions may occur depending on design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a solar cell, characterized in that: include: S1. The N-type silicon wafer is placed in a furnace tube of a diffusion furnace, and nitrogen and oxygen are introduced into the furnace tube to form a first oxide layer on the surface of the silicon wafer; S2. By introducing boron trichloride, oxygen and nitrogen into the furnace tube, a boron diffusion layer is formed on the surface of the first oxide layer away from the silicon wafer; S3. nitrogen is introduced into the furnace tube, and the temperature of the furnace tube is controlled to be a first temperature to promote the oxygen-free boron element; S4. nitrogen and oxygen are introduced into the furnace tube, and the temperature of the furnace tube is controlled to rise from the first temperature to the second temperature to promote the aerobic boron element; S5. Dry oxygen and wet oxygen are alternately introduced into the furnace tube several times to form a second oxide layer; S6. Lowering the temperature in the furnace tube, and continuing to introduce dry oxygen into the furnace tube; S7. Introduce nitrogen into the furnace tube and lower the temperature inside the furnace tube to complete the boron diffusion of the silicon wafer.

2. The method for preparing a solar cell according to claim 1, characterized in that: The step S5 includes: continuously introducing nitrogen and oxygen into the furnace tube to form a dry oxygen atmosphere, and intermittently introducing nitrogen diluted with pure water into the furnace tube to form a wet oxygen atmosphere.

3. The method for preparing a solar cell according to claim 2, characterized in that: In the dry oxygen atmosphere, the flow rate of the nitrogen gas is 600 sccm to 1300 sccm, and the flow rate of the oxygen gas is 20000 sccm to 30000 sccm; In the wet oxygen atmosphere, the flow rate of the nitrogen gas is 1000 sccm to 1500 sccm.

4. The method for preparing a solar cell according to claim 1, characterized in that: In step S5, the temperature of the furnace tube is controlled to be 950° C. to 1040° C., the pressure is controlled to be 600 mbar to 800 mbar, and the time is controlled to be 30 min to 50 min.

5. The method for preparing a solar cell according to claim 1, characterized in that: The step S5 comprises: introducing a first dry oxygen, a first wet oxygen, a second dry oxygen, a second wet oxygen and a third dry oxygen into the furnace tube alternately in sequence for multiple times.

6. The method for preparing a solar cell according to claim 1, characterized in that: In the step S1, the temperature in the furnace tube is controlled to be 790° C. to 870° C., the flow rate of nitrogen is 500 sccm to 4000 sccm, the flow rate of oxygen is 3000 sccm to 6000 sccm, and the processing time is 5 min to 8 min.

7. The method for preparing a solar cell according to claim 1, characterized in that: In step S2, the temperature in the furnace tube is controlled to be 800° C. to 880° C., the pressure is controlled to be 160 mbar to 210 mbar, the flow rate of boron trichloride is controlled to be 350 sccm to 600 sccm, and the flow rate of nitrogen is controlled to be 500 sccm to 4000 sccm.

8. The method for preparing a solar cell according to claim 1, characterized in that: In the step S3, the first temperature is 860° C. to 900° C., the oxygen-free advancement time is 2 min to 8 min, and the flow rate of nitrogen is 500 sccm to 4000 sccm; and / or, In the step S4, the second temperature is 950°C to 1040°C, the flow rate of oxygen is 10000sccm to 25000sccm, and the time of the aerobic advancement is 20min to 30min.

9. The method for preparing a solar cell according to claim 1, characterized in that: In the step S6, the temperature in the furnace tube is controlled to be 930° C. to 980° C., and the flow rate of the dry oxygen is controlled to be 10000 sccm to 30000 sccm.

10. The method for preparing a solar cell according to claim 1, characterized in that: Before step S1, the method further includes: 3500-4500 sccm of nitrogen is introduced into the furnace tube to make the gas pressure in the furnace tube equal to the external gas pressure, and the temperature in the furnace tube is controlled to be 750° C.-780° C.; The pressure in the furnace tube is reduced to 600 mbar to 700 mbar, and at the same time, the temperature of the furnace tube is controlled to rise to 790° C. to 825° C. within 10 to 12 minutes, and the pressure is reduced to 160 mbar to 210 mbar, and the temperature is kept constant for 2 to 5 minutes; After the pressure in the furnace tube is pumped down to 160 mbar to 210 mbar, the air inlet valve and the air exhaust valve are closed, and the furnace tube is vacuum leak tested; and / or, after step S7, the method further includes: removing the second oxide layer, the boron diffusion layer, and the first oxide layer from at least a portion of the surface of the silicon wafer.

Images