Monocrystalline silicon rod and preparation method thereof, silicon wafer and solar cell

Abstract

The embodiment of the invention relates to the field of photovoltaic technology, and provides a single crystal silicon rod and a preparation method thereof, a silicon wafer and a solar cell, so as to improve the radial and axial uniformity of the resistance of the single crystal rod and further improve the efficiency of the solar cell. The preparation method of the silicon single crystal rod comprises an equal-diameter growth process which is sequentially divided into a first stage, a second stage, a third stage and a fourth stage according to the change trend of the pulling speed; the equal-diameter growth process comprises the following steps: gradually increasing the pulling speed from an initial pulling speed to a first pulling speed and gradually increasing the crystal rotation from an initial crystal rotation to a first crystal rotation in a first stage on the basis of a positive correlation relationship between the pulling speed and the crystal rotation; in the second stage, the pulling speed is kept at the first pulling speed, and crystal transition is kept to be first crystal transition; in the third stage, the pulling speed is gradually decreased from the first pulling speed to the second pulling speed, and the crystal transition is gradually decreased from the first crystal transition to the second crystal transition; and in the fourth stage, the pulling speed is kept at the second pulling speed, and the crystal transition is kept to be second crystal transition.

Description

Technical Field

[0001] The present application relates to the field of photovoltaic technology, and in particular to a single crystal silicon rod and a preparation method thereof, a silicon wafer, and a solar cell. Background Art

[0002] Existing single-crystal silicon ingot production processes are primarily designed for producing low-resistance ingots and are not suitable for the high-resistance ingot drawing requirements. High-resistance ingots drawn using existing processes exhibit poor radial and axial resistance uniformity, making them incompatible with back-end cell processing, thereby reducing solar cell efficiency. Summary of the Invention

[0003] The embodiments of the present application provide a single crystal silicon rod and a preparation method thereof, a silicon wafer, and a solar cell to improve the radial and axial uniformity of the resistance of the single crystal rod, thereby improving the efficiency of the solar cell.

[0004] According to some embodiments of the present application, on one hand, a method for preparing a single crystal silicon rod is provided, comprising: a constant diameter growth process, wherein the constant diameter growth process is divided into a first stage, a second stage, a third stage, and a fourth stage according to a change trend of a pulling speed;

[0005] The isodiametric growth process comprises:

[0006] Based on the positive correlation between the pulling speed and the crystal rotation, in the first stage, the pulling speed is gradually increased from the initial pulling speed to the first pulling speed, and the crystal rotation is gradually increased from the initial crystal rotation to the first crystal rotation;

[0007] In the second stage, the pulling speed is maintained at the first pulling speed, and the crystal rotation is maintained at the first crystal rotation;

[0008] In the third stage, the pulling speed is gradually reduced from the first pulling speed to the second pulling speed, and the crystal rotation is gradually reduced from the first crystal rotation to the second crystal rotation;

[0009] In the fourth stage, the pulling speed is maintained at the second pulling speed, and the crystal rotation is maintained at the second crystal rotation.

[0010] According to some embodiments of the present application, the range of the initial crystal rotation is [5 rpm, 7 rpm], and the range of the first crystal rotation is [10 rpm, 12 rpm].

[0011] According to some embodiments of the present application, the difference between the first crystal rotation and the initial crystal rotation accounts for a percentage of the first crystal rotation in the range of [30%, 70%].

[0012] According to some embodiments of the present application, the range of the initial pulling speed is [68 mm / h, 75 mm / h], and the range of the first pulling speed is [100 mm / h, 105 mm / h].

[0013] According to some embodiments of the present application, the isodiametric growth process further includes:

[0014] In the first stage, the crucible rotation is gradually increased from the initial crucible rotation to the first crucible rotation;

[0015] In the second stage, the third stage and the fourth stage, the crucible rotation is maintained as the first crucible rotation.

[0016] According to some embodiments of the present application, the range of the initial crucible rotation is [4 rpm, 6 rpm], and the range of the first crucible rotation is [8 rpm, 10 rpm].

[0017] According to some embodiments of the present application, the isodiametric growth process further includes:

[0018] The furnace pressure is adjusted from the initial furnace pressure to the first furnace pressure in a step-by-step manner, wherein the range of the initial furnace pressure is [5 torr, 7 torr] and the range of the first furnace pressure is [3 torr, 5 torr].

[0019] According to some embodiments of the present application, after the isodiameter growth process, the preparation method further includes: a cooling process;

[0020] The cooling process includes gradually increasing the furnace pressure from the first furnace pressure to the initial furnace pressure.

[0021] According to some embodiments of the present application, before the equal diameter growth process, the preparation method further includes: a shoulder release process;

[0022] The shouldering process includes: adjusting the furnace pressure from a first shouldering furnace pressure to a second shouldering furnace pressure in a step-by-step manner, wherein the first shouldering furnace pressure ranges from [7 torr, 9 torr] and the second shouldering furnace pressure ranges from [5 torr, 7 torr].

[0023] According to some embodiments of the present application, another aspect of the embodiments of the present application provides a single crystal silicon rod formed by the above-mentioned preparation method, wherein the resistance value range of the single crystal silicon rod is [6Ω, 50Ω], and the diameter range of the single crystal silicon rod is [250mm, 400mm].

[0024] According to some embodiments of the present application, another aspect of the embodiments of the present application provides a silicon wafer, wherein the silicon wafer is cut from the above-mentioned single crystal silicon rod, or the silicon wafer is cut from the single crystal silicon rod prepared by the above-mentioned method for making a single crystal silicon rod;

[0025] The resistance value of the silicon wafer is in the range of [6Ω, 50Ω], the thickness of the silicon wafer is in the range of [85μm, 210μm], and the diameter of the silicon wafer is in the range of [250mm, 400mm].

[0026] According to some embodiments of the present application, another aspect of the embodiments of the present application provides a solar cell, including: the above-mentioned silicon wafer; wherein the silicon wafer includes a front side and a back side arranged opposite to each other, and the back side of the silicon wafer is provided with doping regions of different polarities.

[0027] The embodiments of the present application provide a single crystal silicon rod and its preparation method, silicon wafer, and solar cell. In the preparation method of the single crystal silicon rod, in the first stage of the equal diameter growth process, the pulling speed and crystal rotation are gradually increased, and the crystal begins to grow gradually; in the second stage, the pulling speed and crystal rotation are maintained at the maximum value, and the crystal enters a rapid growth period; in the third stage, the pulling speed and crystal rotation are gradually reduced, and the crystal enters a stable growth period; in the fourth stage, the pulling speed and crystal rotation remain unchanged, and the crystal enters a stable ending period. In the first stage, the pulling speed and crystal rotation are gradually increased, which can ensure that the silicon atoms have sufficient time to arrange the lattice correctly, thereby reducing defects such as dislocations and twins. In addition, the low pulling speed can make the distribution of impurities and dopants in the crystal more uniform. In the second stage, the pulling speed and crystal rotation are maintained at the maximum value. The high crystal rotation makes the distribution of impurities and dopants in the radial and axial directions more uniform, and the high pulling speed makes the crystal grow rapidly. In the third stage, as the crystal continues to grow, the amount of molten silicon in the crucible continues to decrease, and the reaction speed to temperature is faster. If the pulling speed and high crystal rotation are still maintained, crystal shaking is likely to occur, and the crystal is prone to defects such as broken wires, cracks or deformation. Therefore, the values ​​of the pulling speed and crystal rotation need to be gradually reduced in the third stage. In the fourth stage, the values ​​of the pulling speed and crystal rotation no longer decrease, but remain unchanged. On the one hand, this ensures the subsequent crystal growth rate, and on the other hand, it ensures the uniformity of the radial and axial distribution of impurities and dopants. According to the growth of the crystal, by setting the pulling speed and crystal rotation with corresponding changing trends at different stages of the equal-diameter growth process, the radial and axial uniformity of the resistance of the single crystal rod can be effectively improved, thereby improving the efficiency of the solar cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise stated, the pictures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the traditional technology, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 A schematic flow chart of a method for preparing a single crystal silicon rod provided in an embodiment of the present application;

[0030] Figure 2 A schematic diagram of a flow chart of an isodiameter growth process provided in an embodiment of the present application;

[0031] Figure 3 A schematic diagram of measuring the axial edge resistance of a crystal ingot provided in an embodiment of the present application;

[0032] Figure 4 For Figure 3 Schematic diagram of measuring the radial resistance of the crystal rod shown. DETAILED DESCRIPTION

[0033] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the specific embodiments of this application and the corresponding drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0034] In the description of the embodiments of the present application, "at least one" means one or more, "at least one layer" means one or more layers, "multiple" means two or more, "multiple layers" means two or more layers, "multiple groups" means two or more groups, and "multiple pieces" means two or more pieces, unless otherwise clearly and specifically defined.

[0035] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0036] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0037] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0038] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "transverse", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the embodiments of the present application. For example, if the device or element in the figure is inverted, then the element described as being "below" or "below" or "below" or "bottom" of other elements or features will be oriented "above" or "top" of the other elements or features. Therefore, the term "below" can cover both above and below orientations depending on the context in which the term is used, which will be obvious to a person skilled in the art. The material can be oriented in other ways (e.g., rotated 90 degrees, inverted, flipped), and the spatially relative descriptors used herein can be interpreted accordingly.

[0039] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise stated, other components are not excluded, and other components may be further included. A second component is formed or provided above or on the first component, or a second component is formed or provided on the surface of the first component, or a second component is formed or provided on one side of the first component. Embodiments in which the first component and the second component are in direct contact may be included, and embodiments in which additional components may be provided between the first component and the second component so that the first component and the second component may not be in direct contact may also be included. For the sake of simplicity and clarity, various components may be arbitrarily drawn in different proportions. In the accompanying drawings, some layers / components may be omitted for simplicity. Unless otherwise specified, a second component is formed or provided on the surface of the first component, which means that the first component is in direct contact with the second component. Among them, the above-mentioned "component" may refer to: layer, film, area, part, structure, etc.

[0040] Single crystal ingots produced using the current crystal pulling process have low resistance (less than 4Ω) and low radial resistance variation (RRV), meeting the requirements of current solar cell applications, such as TOPCON cells (Tunnel Oxide Passivated Contact). However, high-resistance single crystal ingots (greater than or equal to 4Ω) produced using the existing crystal pulling process have poor radial and axial resistance uniformity, making them incompatible with back-end cell processes and, consequently, unable to maximize solar cell efficiency.

[0041] Based on this, an embodiment of the present application provides a method for preparing a single crystal silicon rod, which improves the radial and axial uniformity of the resistance of the single crystal silicon rod by adjusting the parameter settings of the pulling speed and crystal rotation in the equal-diameter growth process.

[0042] The method for preparing a single crystal silicon rod provided in the embodiment of the present application includes: Figure 1 As shown in S20, the equal-diameter growth process, wherein the equal-diameter growth process is divided into the first stage, the second stage, the third stage and the fourth stage in accordance with the change trend of the pulling speed.

[0043] The process of preparing single crystal silicon rods by the Czochralski method includes: silicon material melting process, seeding process, shoulder release process, shoulder rotation process and equal-diameter growth process, among which the equal-diameter growth process is the key process for forming single crystal silicon rods. The parameter design of pulling speed and crystal rotation in this process directly affects the radial and axial uniformity of the resistance of the single crystal silicon rod.

[0044] In the embodiment of the present application, the pulling speed is the crystal pulling speed, which refers to the length of the silicon rod stretched per unit time. The changing trends of the pulling speed include the pulling speed remaining unchanged, the pulling speed gradually increasing, and the pulling speed gradually decreasing. In the equal-diameter growth process, a pulling speed that is too fast will cause stress inside the crystal, affecting the quality of the crystal rod; a pulling speed that is too slow will cause the growth time to be extended, affecting production efficiency. Therefore, in the equal-diameter growth process, it is very important to set a suitable pulling speed. In the equal-diameter growth process of the embodiment of the present application, the changing trends of the pulling speed are sequentially set to the pulling speed gradually increasing → the pulling speed remaining unchanged → the pulling speed gradually decreasing → the pulling speed remaining unchanged. According to the changing trends of the pulling speed, the equal-diameter growth process is divided into the first stage, the second stage, the third stage, and the fourth stage; wherein, the first stage corresponds to the gradually increasing pulling speed, the second stage corresponds to the unchanged pulling speed, the third stage corresponds to the gradually decreasing pulling speed, and the fourth stage corresponds to the unchanged pulling speed.

[0045] refer to Figure 2 As shown, S20, the equal diameter growth process includes:

[0046] S201 , based on the positive correlation between the pulling speed and the crystal rotation, in the first stage, gradually increasing the pulling speed from the initial pulling speed to the first pulling speed, and gradually increasing the crystal rotation from the initial crystal rotation to the first crystal rotation.

[0047] S202. In the second stage, the pulling speed is maintained at the first pulling speed, and the crystal rotation is maintained at the first crystal rotation.

[0048] S203 , in the third stage, gradually reducing the pulling speed from the first pulling speed to the second pulling speed, and gradually reducing the crystal rotation from the first crystal rotation to the second crystal rotation.

[0049] S204: In the fourth stage, the pulling speed is maintained at the second pulling speed, and the crystal rotation is maintained at the second crystal rotation.

[0050] The positive correlation between pulling speed and crystal rotation means that the pulling speed and crystal rotation have the same changing trend. That is, in the isodiametric growth process, the changing trend of crystal rotation is set to gradually increase in the first stage, remain unchanged in the second stage, gradually decrease in the third stage, and remain unchanged in the fourth stage.

[0051] In the embodiments of the present application, the term "gradually increasing" refers to a continuous increase according to a preset step size of the drive device. For example, the initial pulling speed L0 gradually increases to the first pulling speed L1, which means that the initial pulling speed L0 continuously increases according to L0+δ1, L0+2δ1, L0+3δ1, L0+4δ1, and so on, until it reaches the first pulling speed L1, where δ1 is the first preset step size of the pull rod drive motor. Similarly, the term "gradually decreasing" refers to a continuous decrease according to a preset step size of the drive device. For example, the first pulling speed L1 gradually decreases to the second pulling speed L2, which means that the first pulling speed L1 continuously decreases according to L1-δ2, L1-2δ2, L1-3δ2, L1-4δ2, and so on, until it reaches the second pulling speed L2, where δ2 is the second preset step size of the pull rod drive motor. The first preset step size and the second preset step size may be the same or different.

[0052] Crystal rotation refers to the rotation speed of the crystal itself. The larger the crystal rotation, the faster the crystal rotates. The centrifugal force and viscous force generated by the rotation can drive the melt to produce forced convection, thereby breaking the asymmetry of natural convection (such as the downflow caused by fast edge heat dissipation), making the dopant (for example: any one or any combination of phosphorus, antimony and arsenic, etc.) more evenly distributed in the radial and axial directions, thereby improving the radial and axial uniformity of the resistance. However, the larger the crystal rotation, the more it will intensify the melt turbulence, resulting in uneven stress distribution inside the crystal, thereby increasing thermal stress and causing cracks or deformation in the crystal during growth. Therefore, in the isodiameter growth process, it is particularly important to set a suitable crystal rotation.

[0053] In the method for preparing a single crystal silicon rod provided in an embodiment of the present application, in the first stage of the equal-diameter growth process, the pulling speed and the crystal rotation are gradually increased, and the crystal begins to grow gradually; in the second stage, the pulling speed and the crystal rotation are maintained at the maximum value, and the crystal enters a rapid growth period; in the third stage, the pulling speed and the crystal rotation are gradually reduced, and the crystal enters a stable growth period; in the fourth stage, the pulling speed and the crystal rotation remain unchanged, and the crystal enters a stable ending period. In the first stage, the pulling speed and the crystal rotation are gradually increased, which can ensure that the silicon atoms have sufficient time to carry out the correct lattice arrangement, thereby reducing defects such as dislocations and twins. In addition, the low pulling speed can make the distribution of impurities and dopants in the crystal more uniform. In the second stage, the pulling speed and the crystal rotation are maintained at the maximum value. The high crystal rotation makes the distribution of impurities and dopants in the radial and axial directions more uniform, and the high pulling speed makes the crystal grow rapidly. In the third stage, as the crystal continues to grow, the amount of molten silicon in the crucible continues to decrease, and the reaction speed to temperature is faster. If the pulling speed and high crystal rotation are still maintained, crystal shaking is likely to occur, and the crystal is prone to defects such as broken wires, cracks or deformation. Therefore, the values ​​of the pulling speed and crystal rotation need to be gradually reduced in the third stage. In the fourth stage, the values ​​of the pulling speed and crystal rotation no longer decrease, but remain unchanged. On the one hand, this ensures the subsequent crystal growth rate, and on the other hand, it ensures the uniformity of the radial and axial distribution of impurities and dopants. According to the growth of the crystal, by setting the pulling speed and crystal rotation with corresponding changing trends at different stages of the equal-diameter growth process, the radial and axial uniformity of the resistance of the single crystal rod can be effectively improved, thereby improving the efficiency of the solar cell.

[0054] The method for preparing a single crystal silicon rod provided in the embodiments of the present application is suitable for preparing a high-resistance single crystal rod (greater than or equal to 4Ω). The high-resistance single crystal rod can be used in solar cells such as BC cells (Back Contact cells).

[0055] In the first stage, the temperature and convection conditions are relatively unstable. If the initial crystal rotation is too large, this instability will be aggravated, resulting in defects such as dislocations and cracks in the crystal. Therefore, it is necessary to set a smaller initial crystal rotation and gradually increase it to the first crystal rotation. In the second stage, the crystal enters a rapid growth period. In order to ensure that impurities and dopants are more evenly distributed in the radial and axial directions, it is necessary to maintain a high crystal rotation. Therefore, the first crystal rotation needs to be set larger. In some embodiments, the range of the initial crystal rotation is [5rpm, 7rpm]. For example, the initial crystal rotation can be 5rpm, 5.5rpm, 6.0rpm, 6.5rpm or 7rpm; the range of the first crystal rotation is [10rpm, 12rpm]. For example, the first crystal rotation can be 10rpm, 10.5rpm, 11rpm, 11.5rpm or 12rpm.

[0056] In some embodiments, the difference between the first crystal rotation and the initial crystal rotation accounts for a percentage of the first crystal rotation in the range of [30%, 70%]. For example, the percentage can be 30%, 40%, 50%, 60%, or 70%. In this case, the initial crystal rotation accounts for a percentage of the first crystal rotation in the range of [30%, 70%]. That is, in the first stage, the initial crystal rotation differs significantly from the first crystal rotation, thereby minimizing the impact on stability in the first stage; while ensuring a higher crystal rotation in the second stage to improve the uniformity of the distribution of impurities and dopants in the radial and axial directions.

[0057] In order to ensure uniform distribution of impurities and dopants in the radial and axial directions in the fourth stage, the second crystal rotation is not set too small. In some embodiments, the second crystal rotation can be greater than the initial crystal rotation.

[0058] In the first stage, the temperature and convection conditions are relatively unstable. If the initial pulling speed is too high, this instability will be exacerbated, resulting in defects such as crystal breakage and uneven distribution of impurities and dopants in the radial and axial directions. Therefore, it is necessary to set a smaller initial pulling speed and gradually increase it to the first pulling speed. In the second stage, the crystal enters a rapid growth period. In order to ensure the crystal growth rate, it is necessary to maintain a high pulling speed. Therefore, the first pulling speed needs to be set relatively high. In some embodiments, the initial pulling speed ranges from [68 mm / h, 75 mm / h]. For example, the initial pulling speed can be 68 mm / h, 69 mm / h, 70 mm / h, 71 mm / h, 72 mm / h, 73 mm / h, 74 mm / h, or 75 mm / h; the first pulling speed ranges from [100 mm / h, 105 mm / h]. For example, the first pulling speed can be 100 mm / h, 101 mm / h, 102 mm / h, 103 mm / h, 104 mm / h, or 105 mm / h.

[0059] Crucible rotation refers to the rotation speed of the crucible. In the isodiametric growth process of single crystal silicon rods, crucible rotation can improve the temperature uniformity of the melt, prevent convection and temperature gradients in the melt, and thus reduce defects and impurities in the crystal. The higher the crucible rotation, the better the uniformity of the melt. In some embodiments, the isodiametric growth process further includes: in the first stage, gradually increasing the crucible rotation from the initial crucible rotation to the first crucible rotation; in the second, third and fourth stages, maintaining the crucible rotation at the first crucible rotation. That is, step S201 also includes gradually increasing the crucible rotation from the initial crucible rotation to the first crucible rotation, and steps S202, S203 and S204 also include maintaining the crucible rotation at the first crucible rotation.

[0060] In the first stage, the crucible rotation gradually increases, and the movement of the solution in the crucible also gradually strengthens, thus preventing violent movement of the solution and further ensuring the stability of crystal growth. In the second, third, and fourth stages, the crucible rotation remains unchanged, which can improve the uniformity of the solution in the crucible. At the same time, in conjunction with the crystal rotation (the two rotate in opposite directions), the distribution of impurities and dopants in the radial and axial directions can be more uniform.

[0061] In any stage of the isodiametric growth process, the ratio of crucible rotation to crystal rotation is in the range of [0.7, 1.6]. For example, the ratio of crucible rotation to crystal rotation is 0.7, 0.8, 0.9, 1.0, 1.2, 1.4 or 1.6. By ensuring that the ratio of crucible rotation to crystal rotation is within the range of 0.7-1.6, the radial distribution uniformity of impurities and dopants can be further improved, thereby improving the radial uniformity of resistance. When the radial distribution uniformity of impurities and dopants is satisfied, there will be no faults in the distribution of impurities and dopants, and the impurity and dopant content of the formed crystal liquid surface will not fluctuate, thereby making the axial resistance more uniform and shortening the difference in axial resistance, that is, improving the axial uniformity of resistance, and ultimately controlling the head-to-tail resistance ratio of the single crystal silicon rod to be between 1.2 and 3.1.

[0062] In the first stage, if the initial crucible rotation is too large, the fluctuation of the molten silicon liquid level in the crucible will be aggravated, which will affect the growth quality of the crystal. Therefore, it is necessary to set a smaller initial crucible rotation and gradually increase it to the first crucible rotation. In the second, third and fourth stages, in order to improve the radial distribution uniformity of impurities and dopants, it is necessary to maintain a relatively high first crucible. In some embodiments, the range of the initial crucible rotation is [4rpm, 6rpm]. For example, the initial crucible rotation can be 4rpm, 4.5rpm, 5rpm, 5.5rpm or 6rpm; the range of the first crucible rotation is [8rpm, 10rpm]. For example, the first crucible rotation can be 8rpm, 8.5rpm, 9rpm, 9.5rpm or 10rpm.

[0063] Furnace pressure refers to the pressure inside the single crystal furnace. In the isodiametric growth process of single crystal silicon rods, changes in furnace pressure will affect the temperature distribution and gas phase transmission of the melt, thereby affecting the growth quality, shape and oxygen content of the crystal. Therefore, it is particularly important to set the furnace pressure reasonably. In some embodiments, the isodiametric growth process further includes: adjusting the furnace pressure from the initial furnace pressure to the first furnace pressure in a step-by-step manner, wherein the initial furnace pressure is in the range of [5 torr, 7 torr], for example, the initial furnace pressure is 5 torr, 5.5 torr, 6 torr, 6.5 torr or 7 torr, and the first furnace pressure is in the range of [3 torr, 5 torr], for example, the first furnace pressure is 3 torr, 3.5 torr, 4 torr, 4.5 torr or 5 torr.

[0064] In the embodiment of the present application, the step-by-step decreasing method refers to: the furnace pressure decreases according to the rule of remaining unchanged → decreasing → remaining unchanged → decreasing → remaining unchanged...; there is no limit on the number of steps set here; for example, taking the furnace pressure as an example of a two-step step-by-step decreasing method, the furnace pressure remains at furnace pressure P1 in the first period, decreases from furnace pressure P1 to furnace pressure P2 in the second period, remains at furnace pressure P2 in the third period, decreases from furnace pressure P2 to furnace pressure P3 in the fourth period, and remains at furnace pressure P3 in the fifth period. Among them, the dividing point between the first period and the second period, and the dividing point between the second period and the third period are all set within the aforementioned first stage, and the dividing point between the third period and the fourth period, and the dividing point between the fourth period and the fifth period are all set within the aforementioned second stage. That is, corresponding to the aforementioned first stage, the furnace pressure is maintained at P1 → the furnace pressure P1 is gradually reduced to P2 → the furnace pressure is maintained at P2; corresponding to the aforementioned second stage, the furnace pressure is maintained at P2 → the furnace pressure P2 is gradually reduced to P3 → the furnace pressure is maintained at P3; corresponding to the aforementioned third and fourth stages, the furnace pressure is always maintained at P3.

[0065] In the equal-diameter growth process of single crystal silicon rods, the furnace pressure decreases step by step from the initial furnace pressure to the first furnace pressure. On the one hand, this can control the growth rate of the crystal in the initial stage and ensure its growth quality; on the other hand, the step-by-step decrease in furnace pressure can slow down the severity of furnace pressure changes and provide a stable crystal growth environment; on the other hand, in the later stage of crystal growth, the furnace pressure value is relatively small, which can increase the convection intensity at the melt interface, help the volatilization of oxygen, thereby reducing the oxygen content of the single crystal silicon rod and avoiding the problem of high oxygen content caused by higher crucible rotation in the later stage.

[0066] After the isodiameter growth process is completed, the single crystal silicon rod is still in a high temperature state. In order to prevent the single crystal silicon rod from being deformed or damaged during subsequent processing, a cooling process is required. Figure 1 As shown, after the equal-diameter growth process S20, the preparation method further includes: S30, a cooling process.

[0067] S30, the cooling process, includes the following steps: S301, gradually increasing the furnace pressure from the first pressure to the initial pressure. During the constant diameter growth process, the furnace pressure eventually decreases to the first pressure, which is relatively low. During the cooling process, the single crystal silicon rods are pulled out of the molten silicon liquid, increasing the evaporation area of ​​the molten silicon liquid and causing the oxygen content above the molten silicon liquid to rise. To reduce silicon oxide volatilization, the furnace pressure needs to be gradually increased from the first pressure to the initial pressure.

[0068] refer to Figure 1As shown, before the step S20, the equal diameter growth step, the preparation method further includes: S10, the shoulder release step. The step S10, the shoulder release step, includes: S101, adjusting the furnace pressure from a first shoulder release furnace pressure to a second shoulder release furnace pressure in a stepwise decreasing manner. The first shoulder release furnace pressure ranges from [7 Torr, 9 Torr]. For example, the first shoulder release furnace pressure can be 7 Torr, 7.5 Torr, 8 Torr, 8.5 Torr, or 9 Torr. The second shoulder release furnace pressure ranges from [5 Torr, 7 Torr]. For example, the second shoulder release furnace pressure can be 5 Torr, 5.5 Torr, 6 Torr, 6.5 Torr, or 7 Torr. During the shoulder release step, the furnace pressure is stepped down from the first shoulder release furnace pressure to the second shoulder release furnace pressure. This ensures the initial shoulder release length and maintains a lower furnace pressure in the later stages, effectively reducing the oxygen content and providing a low-oxygen environment for the next shoulder release step.

[0069] The main purpose of the shouldering process is to change the diameter. In order to avoid oxygen entering the crystal as much as possible, low crystal rotation (5rpm-7rpm) and low crucible rotation (3.5rpm-5.5rpm) are adopted, and the crystal rotation and crucible rotation remain unchanged; at the same time, low crystal rotation can make the crystallization interface convex toward the solution, which is more conducive to crystal pulling.

[0070] The following compares the parameter settings for the shoulder release and equal-diameter growth processes provided in the examples of this application with those provided in the comparative examples. Table 1 shows the parameter settings for the shoulder release process in the comparative example, and Table 2 shows the parameter settings for the equal-diameter growth process in the comparative example. Table 3 shows the parameter settings for the shoulder release process in the examples of this application, and Table 4 shows the parameter settings for the equal-diameter growth process in the examples of this application.

[0071] Table 1

[0072]

[0073] It can be concluded from Table 1 that in the comparative example, during the shoulder release process, the furnace pressure, crystal rotation and crucible rotation remained unchanged.

[0074] Table 2

[0075]

[0076] It can be concluded from Table 2 that in the comparative example, in the isodiameter growth process, the crystal rotation, crucible rotation and furnace pressure remain unchanged.

[0077] Table 3

[0078]

[0079] It can be concluded from Table 3 that in the embodiment of the present application, in the shouldering process, the crystal rotation is maintained at 6 rpm, the crucible rotation is maintained at 4.5 rpm, and the furnace pressure is gradually reduced from 8 torr in the first shouldering process to 6 torr in the second shouldering process.

[0080] Table 4

[0081]

[0082] It can be concluded from Table 4 that in the embodiment of the present application, in the isodiametric growth process, in the first stage, the pulling speed is gradually increased from the initial pulling speed of 73 mm / h to the first pulling speed of 103 mm / h, the crystal rotation is gradually increased from the initial crystal rotation of 6 rpm to the first crystal rotation of 11 rpm, the crucible rotation is gradually increased from the initial crucible rotation of 5 rpm to the first crucible rotation of 9 rpm, and the furnace pressure is gradually decreased from the initial furnace pressure of 6 torr to 5 torr; in the second stage, the pulling speed is maintained at the first pulling speed of 103 mm / h, the crystal rotation is maintained at the first crystal rotation of 11 rpm, the crucible rotation is maintained at the first crucible rotation of 9 rpm, and the furnace pressure is increased from 5 to After rr decreases to the first furnace pressure of 4torr, it is maintained at the first furnace pressure of 4torr; in the third stage, the pulling speed gradually decreases from the first pulling speed of 103mm / h to the second pulling speed of 97mm / h, the crystal rotation gradually decreases from the first crystal rotation of 11rpm to the second crystal rotation of 9rpm, the crucible rotation continues to be maintained at 9rpm of the first crucible rotation, and the furnace pressure continues to be maintained at the first furnace pressure of 4torr; in the fourth stage, the pulling speed is maintained at the second pulling speed of 97mm / h, the crystal rotation is maintained at 9rpm of the second crystal rotation, the crucible rotation continues to be maintained at 9rpm of the first crucible rotation, and the furnace pressure continues to be maintained at the first furnace pressure of 4torr.

[0083] refer to Figure 3 As shown, along the axial direction of the single crystal silicon rod 1, the axial edge resistances r1 to r5 are detected respectively; Figure 4 As shown, radial resistances R0 to R6 are detected along the radial direction of the single crystal silicon rod, respectively. 4 points are measured in each concentric circle and the average value is calculated to obtain the corresponding radial resistance. Figure 4 The diameter of the medium single crystal silicon rod is 280mm.

[0084] The single crystal silicon rods prepared in the comparative example and the single crystal silicon rods prepared in the embodiment of the present application were respectively Figure 3 and Figure 4 The test is performed as shown, thus obtaining:

[0085] In the single crystal silicon rod prepared in the comparative example, the concentric circle resistances corresponding to radial directions R0 to R6 were 7.3Ω, 8.2Ω, 9.4Ω, 11Ω, 12.7Ω, 14.1Ω, and 15.4Ω, respectively. The axial edge resistances R1 to R5 were 23.3Ω, 18.6Ω, 13.1Ω, 10.5Ω, and 8.1Ω, respectively. The difference between the radial resistances R6 and R0 was 8.1Ω, and the difference between the axial edge resistances R1 and R5 was 15.2Ω.

[0086] In the single crystal silicon rods prepared using the examples of the present application, the concentric circle resistances corresponding to radial directions R0 to R6 were 8.6Ω, 9.1Ω, 9.9Ω, 10.8Ω, 11.7Ω, and 12.8Ω, respectively, and the axial edge resistances r1 to r5 were 22.2Ω, 18.1Ω, 14.2Ω, 11.8Ω, and 9.6Ω, respectively. The difference between the radial resistances R6 and R0 was 4.2Ω, and the difference between the axial edge resistances r1 and r5 was 12.6Ω. Compared to the comparative example, the single crystal silicon rods prepared using the examples of the present application had a maximum radial resistance difference reduced by 48%, and a maximum axial resistance difference reduced by 17%, significantly improving both radial and axial uniformity.

[0087] The present application also provides a single crystal silicon rod, which is formed by any one of the methods for preparing a single crystal silicon rod provided in the above embodiments of the present application, wherein the resistance value of the single crystal silicon rod is in the range of [6Ω, 50Ω]. For example, the resistance value of the single crystal silicon rod is 6Ω, 10Ω, 13Ω, 18Ω, 20Ω, 25Ω, 30Ω, 35Ω, 40Ω, 45Ω or 50Ω, and the diameter of the single crystal silicon rod is in the range of [250mm, 400mm]. For example, the diameter of the single crystal silicon rod is The length of the monocrystalline silicon ingot is 250mm, 253mm, 256mm, 260mm, 270mm, 280mm, 290mm, 300mm, 320mm, 340mm, 360mm, 380mm, or 400mm. The head-to-tail resistance ratio of the monocrystalline silicon ingot is in the range of [1.2, 3.1]. For example, the head-to-tail resistance ratio of the monocrystalline silicon ingot is 1.2, 1.5, 1.8, 2.0, 2.3, 2.5, 2.7, 2.9, 3.0, or 3.1. This monocrystalline silicon ingot is a high-resistance, large-size ingot that can be used in solar cells such as BC cells. Its radial and axial resistance uniformity is good, which can improve the efficiency of solar cells.

[0088] An embodiment of the present application further provides a silicon wafer, which is cut from the single crystal silicon rod provided in the above embodiment, or the silicon wafer is cut from the single crystal silicon rod prepared by any of the methods for making a single crystal silicon rod provided in any of the above embodiments.

[0089] The resistance value range of the silicon wafer is [6Ω, 50Ω]. For example, the resistance value of the silicon wafer is 6Ω, 10Ω, 13Ω, 18Ω, 20Ω, 25Ω, 30Ω, 35Ω, 40Ω, 45Ω or 50Ω; the thickness range of the silicon wafer is [85μm, 210μm]. For example, the thickness of the silicon wafer is 85μm, 90μm, 95μm, 100μm, 105μm, 110μm, 120μm, 130μm, 140μm The silicon wafer diameter range is [250mm, 400mm]. For example, the diameter of the silicon wafer is 250mm, 253mm, 256mm, 260mm, 270mm, 280mm, 290mm, 300mm, 320mm, 340mm, 360mm, 380mm, or 400mm. This high-resistance, large-size silicon wafer can be used in solar cells such as BC cells. Its radial and axial resistance uniformity can improve the efficiency of solar cells.

[0090] An embodiment of the present application further provides a solar cell, comprising: the silicon wafer provided in the aforementioned embodiment; wherein the silicon wafer comprises a front side and a back side arranged opposite to each other, and the back side of the silicon wafer is provided with doping regions of different polarities.

[0091] The type of the solar cell is a back contact cell, which includes but is not limited to an interdigitated back contact solar cell (IBC), a tunnel back contact solar cell (TBC) or a heterojunction back contact solar cell (HBC).

[0092] The silicon wafer is an N-type silicon wafer, that is, it is doped with an N-type doping element. The N-type doping element can be any one or any combination of the fifth main group elements such as phosphorus (P), bismuth (Bi), antimony (Sb) or arsenic (As).

[0093] The back of the N-type silicon wafer is doped to form alternating n+ doped and p+ doped regions on the back surface, so that the back of the solar cell has both positive and negative electrodes, allowing all grid lines to be completely set on the back of the solar cell.

[0094] Those skilled in the art will appreciate that the above-described embodiments are specific examples for implementing the present application, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the present application. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined in the claims.

Claims

1. A method for preparing a single crystal silicon rod, characterized in that: include: The equal diameter growth process is divided into a first stage, a second stage, a third stage and a fourth stage according to the change trend of the pulling speed; The isodiametric growth process comprises: Based on the positive correlation between the pulling speed and the crystal rotation, in the first stage, the pulling speed is gradually increased from the initial pulling speed to the first pulling speed, and the crystal rotation is gradually increased from the initial crystal rotation to the first crystal rotation; In the second stage, the pulling speed is maintained at the first pulling speed, and the crystal rotation is maintained at the first crystal rotation; In the third stage, the pulling speed is gradually reduced from the first pulling speed to the second pulling speed, and the crystal rotation is gradually reduced from the first crystal rotation to the second crystal rotation; In the fourth stage, the pulling speed is maintained at the second pulling speed, and the crystal rotation is maintained at the second crystal rotation.

2. The preparation method according to claim 1, characterized in that The range of the initial crystal rotation is [5 rpm, 7 rpm], and the range of the first crystal rotation is [10 rpm, 12 rpm].

3. The preparation method according to claim 1, characterized in that The difference between the first crystal rotation and the initial crystal rotation accounts for a percentage of the first crystal rotation in the range of [30%, 70%].

4. The preparation method according to claim 1, characterized in that The range of the initial pulling speed is [68 mm / h, 75 mm / h], and the range of the first pulling speed is [100 mm / h, 105 mm / h].

5. The preparation method according to any one of claims 1 to 4, characterized in that The isodiametric growth process further comprises: In the first stage, the crucible rotation is gradually increased from the initial crucible rotation to the first crucible rotation; In the second stage, the third stage and the fourth stage, the crucible rotation is maintained as the first crucible rotation.

6. The preparation method according to claim 5, characterized in that The range of the initial crucible rotation is [4 rpm, 6 rpm], and the range of the first crucible rotation is [8 rpm, 10 rpm].

7. The preparation method according to any one of claims 1 to 4, characterized in that The isodiametric growth process further comprises: The furnace pressure is adjusted from the initial furnace pressure to the first furnace pressure in a step-by-step manner, wherein the range of the initial furnace pressure is [5 torr, 7 torr] and the range of the first furnace pressure is [3 torr, 5 torr].

8. The preparation method according to claim 7, characterized in that After the isodiameter growth step, the preparation method further includes: a cooling step; The cooling process includes gradually increasing the furnace pressure from the first furnace pressure to the initial furnace pressure.

9. The preparation method according to claim 1, characterized in that Before the equal diameter growth step, the preparation method further includes: a shoulder release step; The shouldering process includes: adjusting the furnace pressure from a first shouldering furnace pressure to a second shouldering furnace pressure in a step-by-step manner, wherein the range of the first shouldering furnace pressure is [7 torr, 9 torr], and the range of the second shouldering furnace pressure is [5 torr, 7 torr].

10. A single crystal silicon rod, characterized in that: The manufacturing method according to any one of claims 1 to 9 is used, wherein the resistance value of the single crystal silicon rod is in the range of [6Ω, 50Ω], and the diameter of the single crystal silicon rod is in the range of [250mm, 400mm].

11. A silicon wafer, characterized in that: The silicon wafer is cut from the single crystal silicon rod according to claim 10, or the silicon wafer is cut from the single crystal silicon rod prepared by the method for preparing a single crystal silicon rod according to any one of claims 1 to 9; The resistance value of the silicon wafer is in the range of [6Ω, 50Ω], the thickness of the silicon wafer is in the range of [85μm, 210μm], and the diameter of the silicon wafer is in the range of [250mm, 400mm].

12. A solar cell, characterized in that: include: The silicon wafer according to claim 11; The silicon wafer includes a front side and a back side that are arranged opposite to each other, and the back side of the silicon wafer is provided with doping regions with different polarities.

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