Silicon surface selective heavy doping method, photovoltaic cell and preparation method thereof

Abstract

The invention relates to the technical field of photovoltaics, and discloses a silicon surface selective heavy doping method, a photovoltaic cell and a preparation method of the photovoltaic cell. The method comprises the following steps: carrying out diffusion doping on the surface of a crystalline silicon wafer by adopting a light doping process so as to sequentially form a light doping structure and doped silica glass on the silicon wafer; removing the doped silica glass to expose the lightly doped structure; short-wave high-energy laser with the wavelength smaller than or equal to 355 nm and the energy density ranging from 110 mJ / cm < 2 > to 140 mJ / cm < 2 > is adopted for scanning the lightly-doped structure on the shallow surface layer of the metallized area of the silicon wafer, crystalline silicon in the lightly-doped structure on the shallow surface layer is made to be non-crystallized, and the crystalline silicon structure is still reserved in the non-metallized area; and annealing to activate the doping atoms in the process of recrystallizing the amorphous silicon into crystalline silicon, so that the doping concentration of the shallow surface layer of the metallized region is improved compared with that of the non-metallized region, and the selective emitter is prepared. According to the method, simultaneous optimization of contact and compounding can be realized, and the battery efficiency is further improved.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic technology, specifically to a method for selective heavy doping of silicon surfaces, a photovoltaic cell, and a method for its fabrication. Background Technology

[0002] A solar cell is essentially a large-area PN junction. Donor impurities are doped onto one side of the substrate material to form an n-type (negative) emitter; acceptor impurities are doped onto the other side to form a p-type (positive) structure. Taking crystalline silicon solar cells as an example, boron (B) is typically doped to form a p-emitter, and phosphorus (P) to form an n-emitter. The doping concentration directly affects the performance of the solar cell. TOPCon (Tunnel Oxide Passivated Contact Solar Cell) solar cells are currently the most widely used crystalline silicon solar cell structure. TOPCon cells dope boron into the silicon bulk region on the front side through high-temperature thermal diffusion, forming a p+ emitter. When illuminated, the crystalline silicon solar cell absorbs photons, forming positively charged holes and negatively charged electrons. Under the influence of a built-in electric field, the holes and electrons are separated, and finally connected to an external load through metal grid lines (metal electrodes) on the surface, forming a current loop.

[0003] However, crystalline silicon solar cells face a problem in industrial production: because silicon is a semiconductor material, resistance is generated when metal grid lines (silver, copper, aluminum, etc.) are connected to silicon. To better form ohmic contacts, the metal electrode contact region (i.e., the metallized region) is typically heavily doped to increase the metallicity of the silicon material in this region and reduce resistance. However, at the same time, the increased impurity atoms introduced by heavy doping lead to increased recombination within the cell, which in turn degrades cell performance.

[0004] 1 / τ surforbulk =1 / τ rad +1 / τ sRH +1 / τ Auger

[0005] As can be seen from the above formula, the composite of the surface or volume region (τ) surforbulk ), by radiation recombination (τ) rad ), SRH complex (τ) SRH ) and Auger complex (τ) AugerIt consists of three parts. Radiative recombination is the reverse of light absorption, where electrons return from a high-energy state to a lower-energy state, releasing light energy. Auger recombination is the reverse of collisional ionization; electrons and holes recombine, releasing excess energy, which is absorbed by another electron. This electron then relaxes back to its original energy state and releases a phonon. The magnitude of Auger recombination increases with the concentration of the doped material. Shockley-Reed-Hall (SRH) recombination is a non-radiative process, typically releasing energy as heat. The SRH recombination rate is positively correlated with the excess carrier concentration. Therefore, the ideal doping effect for a crystalline silicon solar cell should be that the shallow surface layer of the silicon material in contact with the metal electrode has a very high doping concentration, but the doping concentration in the corresponding shallow interior of this region decreases sharply to reduce the recombination rate.

[0006] However, the actual experimental situation is as follows Figure 1 As shown, Figure 1 The curve shown is a typical boron diffusion doping concentration-depth (ECV) curve. It can be seen that the maximum boron doping concentration, 1.61E19cm, is found at a depth of 0.2µm in the silicon material. -3 At a depth of 0.4 μm in silicon material, a density of 1.25E19 cm is still maintained. -3 The boron doping concentration.

[0007] Even with the use of light doping techniques such as SE or LECO in current production, experiments have shown that the maximum doping concentration of silicon material is still 1.38E19cm to ensure the contact performance between the metal gate and the silicon semiconductor. -3 At 0.4 μm, it still maintains a strength of 1.06E19 cm. -3 doping concentration (e.g.) Figure 2 (As shown).

[0008] Laser Selective Emitter (SE) technology (as shown in CN120826051A) uses laser energy to propel the dopant source in phosphorus silicon glass (PSG) or borosilicate glass (BSG) a second time, so as to form a heavily doped region (p++) in the laser region and a lightly doped region (p+) in the unlased region.

[0009] LECO (Laser Enhanced Contact Optimization) is a key process widely used in high-efficiency crystalline silicon solar cells (especially TOPCon cells) in recent years. Its core principle is to optimize the contact performance between the metal electrode and the silicon substrate by precisely localizing energy input with a laser, thereby improving cell efficiency.

[0010] Both SE (Self-Doped) and LECO (Low-Doped Silicon) technologies can achieve light doping in the non-metallized regions and heavy doping in the metallized regions. However, neither technology can guarantee light doping within the metallized regions of silicon. This is because current doping methods rely on concentration gradients. To achieve heavy doping at the surface of the metal-silicon semiconductor contact in the metallized region, the doping concentration within the metallized region of the silicon semiconductor must increase. For solar cells, the performance of the metallized region has a greater impact on the final performance of the cell. Therefore, existing technologies struggle to balance heavy doping on the shallow surface of the silicon metallized region with light doping within it, thus hindering the achievement of both excellent contact performance and a low recombination rate, which in turn impedes further improvements in the performance of crystalline silicon solar cells. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for selective heavy doping of silicon surfaces, a photovoltaic cell and its preparation method.

[0012] Based on this, the present invention discloses a method for selective heavy doping of silicon surfaces, comprising the following steps:

[0013] S1. A light doping process is used to diffusely dope the surface of a silicon wafer to sequentially form a lightly doped structure and doped silicon glass on the silicon wafer; the silicon wafer is crystalline silicon.

[0014] S2. Remove the doped silicon glass to expose the lightly doped structure;

[0015] S3. A short-wavelength high-energy laser is used to scan the lightly doped structure of the shallow surface layer of the metallized region of the silicon wafer, so that the crystalline silicon in the lightly doped structure of the shallow surface layer becomes amorphous, while the non-metallized region of the silicon wafer retains the crystalline silicon structure.

[0016] The short-wavelength high-energy laser has a wavelength less than or equal to 355 nm and an energy density of 110-140 mJ / cm². 2 ;

[0017] S4. Annealing transforms amorphous silicon into crystalline silicon. During the recrystallization of amorphous silicon into crystalline silicon, doped atoms are activated, increasing the doping concentration of the shallow layer of the metallized region compared to the non-metallized region, thus producing a selective emitter.

[0018] Preferably, in step S1, boron diffusion doping is performed on the front side of the silicon wafer; the temperature of the light doping process is 800-900℃, the time is 40-70min, the doping source is a boron source, the boron source flow rate is 200-250sccm, and the oxygen flow rate is 300-450sccm.

[0019] More preferably, in step S1, the light doping process includes a pre-oxygen process, a deposition process, a propulsion process, and a post-oxygen process performed sequentially; the oxygen flow rate in the pre-oxygen process is greater than or equal to 300 sccm.

[0020] More preferably, in step S1, the boron source flow rate during the deposition process is less than or equal to 250 sccm.

[0021] Preferably, in step S3, the laser power of the short-wavelength high-energy laser is 0.50-0.60W; the shallow layer is the subsurface of the silicon wafer, located at a depth of 30-60nm from the surface of the silicon wafer.

[0022] More preferably, in step S3, the short-wavelength high-energy laser is a 355nm ultraviolet picosecond laser with a laser power of 0.58W and a laser energy density of 131mJ / cm². 2 .

[0023] Preferably, in step S4, the annealing temperature is 900-950℃ and the time is 30-50 minutes;

[0024] After annealing, the peak doping concentration in the shallow layer of the metallized region is 1.20E19cm. -3 -1.50E19cm -3 The doping concentration within the silicon bulk region beneath the shallow layer of the metallized region rapidly decreases to zero at a depth of 0.6-0.8 μm, while the peak doping concentration of the lightly doped structure in the non-metallized region is 1.00E19 cm⁻¹. -3 -1.45E19cm -3 .

[0025] The present invention also discloses a method for preparing a photovoltaic cell, which includes the method for selective heavy doping of silicon surface described above; the photovoltaic cell is a crystalline silicon solar cell.

[0026] Preferably, a method for preparing a photovoltaic cell according to the present invention, wherein the photovoltaic cell is a TOPCon cell, includes the following preparation steps:

[0027] Step 1: Using a light doping process, diffusion doping is performed on one surface of the texturized silicon wafer to sequentially form a lightly doped structure and doped silicon glass on the silicon wafer; the silicon wafer is crystalline silicon.

[0028] Step 2: Perform single-sided acid cleaning and polishing on the other surface of the silicon wafer, and at the end of the polishing process, use double-sided acid cleaning to remove the doped silicon glass to expose the lightly doped structure.

[0029] Step 3: Scan the lightly doped structure of the shallow surface layer of the metallized region of the silicon wafer using a short-wavelength high-energy laser to amorphize the crystalline silicon in the lightly doped structure of the shallow surface layer, while the non-metallized region of the silicon wafer retains the crystalline silicon structure; the wavelength of the short-wavelength high-energy laser is less than or equal to 355 nm, and its energy density is 110-140 mJ / cm². 2 ;

[0030] Step 4: Sequentially prepare a tunneling layer and a doped amorphous silicon layer on the other surface of the silicon wafer;

[0031] Step 5: Annealing, which converts amorphous silicon into crystalline silicon. During the recrystallization of amorphous silicon into crystalline silicon, doping atoms are activated, which increases the doping concentration of the shallow layer of the metallized region compared to the non-metallized region, so as to form a selective emitter on one surface of the silicon wafer and convert the doped amorphous silicon layer on the other surface of the silicon wafer into a doped polycrystalline silicon layer.

[0032] Step 6: After annealing, the silicon wafer is sequentially cleaned, and passivation films on the front and back sides are deposited. Then, passivation films on the front and back sides of the laser-opened metallized areas are deposited, and metal electrodes on the front and back sides are electroplated to obtain the TOPCon cell.

[0033] The present invention also discloses a photovoltaic cell, which is prepared by the photovoltaic cell preparation method described above in the present invention.

[0034] Compared with the prior art, the present invention has at least the following beneficial effects:

[0035] This invention discloses a method for selective heavy doping of silicon surfaces. Following the light doping process in step S1 and the removal of the front-side BSG in step S2 to expose the lightly doped structure, step S3 employs a short-wavelength high-energy laser to scan the lightly doped structure in the shallow layer of the metallized region, causing the crystalline silicon in the shallow layer to become amorphous. Then, through the synergistic effect of the short-wavelength high-energy laser in step S3 and the annealing process in step S4, the doped atoms in the lightly doped structure in the shallow layer are activated. This simultaneously increases the doping concentration in the shallow layer of the metallized region during the recrystallization of the amorphous silicon during annealing, resulting in a heavily doped structure in the shallow layer of the metallized region. This reduces the contact resistance between the metal electrode and the heavily doped structure, improves contact performance, and reduces metal-to-metal recombination. Meanwhile, the doping concentration inside the silicon bulk region beneath the shallow layer of the metallized region is extremely low (because the short-wavelength laser in step S3 is absorbed and rapidly attenuated when it enters the shallow layer of the metallized region of the silicon wafer; therefore, the metallized region affected by the short-wavelength high-energy laser is limited to a shallow layer within a few tens of nanometers), further reducing recombination inside the silicon bulk region beneath the shallow layer of the metallized region.

[0036] Therefore, the method of the present invention, through the synergistic cooperation of steps S1-S4, can simultaneously achieve the goal of reducing both the heavy doping of the shallow surface layer of the metallized region and the lighter doping of the silicon bulk region below the shallow surface layer of the metallized region, thereby reducing both the recombination in the shallow surface layer of the metallized region and the recombination in the silicon bulk region of the silicon wafer; and the heavy doping of the shallow surface layer can reduce the contact resistance and improve the contact performance; thus achieving simultaneous optimization of contact and recombination, it can effectively solve the problem that the shallow surface layer of the metallized region on the surface of the solar cell needs to be heavily doped while the silicon bulk region inside the metallized region needs to be lightly doped, which helps to further improve the cell efficiency without adding additional equipment. Attached Figure Description

[0037] Figure 1 This is a typical boron diffusion doping concentration-depth (ECV) curve of a silicon wafer surface after existing boron diffusion doping.

[0038] Figure 2 This is a graph showing the concentration-depth of boron diffusion doping after treatment with existing light doping techniques combined with LECO technology.

[0039] Figure 3 This is a graph showing the test data of the battery open-circuit voltage after adjusting the laser energy density in step 3 of the present invention.

[0040] Figure 4 This is a graph showing the boron diffusion doping concentration versus doping depth in the metallized region on the front side of the battery in Example 1.

[0041] Figure 5 The graph shows the boron diffusion doping concentration versus doping depth in the metallized region on the front side of the battery in Comparative Example 1.

[0042] Figure 6 This is a comparison chart of the battery composite density J0 test data for Example 1 and Comparative Example 1.

[0043] Figure 7 This is a comparison chart of battery efficiency Eta test data for Example 1 and Comparative Example 1. Detailed Implementation

[0044] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] A method for preparing a photovoltaic cell, taking a TOPCon cell as an example, includes the following preparation steps:

[0046] Step 1: Using a light doping process, the front surface of the texturized silicon wafer is diffused and doped (including but not limited to boron diffusion doping) to sequentially form a lightly doped structure and BSG (borosilicate glass) on the front surface of the silicon wafer.

[0047] In step 1, the temperature of the light doping process is 800-900℃, the time is 40-70 min, the boron source flow rate is 200-250 sccm, and the oxygen flow rate is 300-450 sccm.

[0048] The boron diffusion doping in step 1 is to incorporate impurity atoms (such as boron atoms) into the silicon wafer. As described in the background art, the ideal doping effect for a crystalline silicon solar cell should be that the shallow surface layer of the silicon wafer in contact with the metal electrode (i.e., the metallized region) is heavily doped, while the doping level in the silicon bulk region below the shallow surface layer of the metallized region is extremely low (i.e., balancing the heavy doping of the shallow surface layer of the metallized region with the extremely light doping in the silicon bulk region of the metallized region).

[0049] However, conventional boron diffusion doping processes are limited by the thermal diffusion mechanism. The doping amount of silicon materials (such as silicon wafers) is achieved according to the concentration gradient. That is, in order to maintain a certain depth of heavy doping concentration, the doping concentration at a deeper depth (i.e., inside the silicon bulk region) will not decrease immediately, but will gradually decrease, leading to an increase in recombination in the silicon bulk region.

[0050] The boron diffusion doping process mainly includes four processes performed sequentially: pre-oxygenation, deposition, propulsion, and post-oxygenation.

[0051] Pre-oxidation: An oxide layer (such as a silicon oxide layer, which will later form part of the BSG and is cleaned off in step 2 below) is formed on the surface of the silicon wafer to allow dopants to enter the silicon bulk region more uniformly.

[0052] Deposition: A layer of doped atoms is deposited on the oxide layer to form a boron-rich layer (BRL).

[0053] Advancement: Boron atoms of BRL are pushed into the silicon bulk region through high temperature to form a lightly doped structure.

[0054] Post-oxidation: BRL is oxidized with a large amount of oxygen to borosilicate glass, which is easily removed by acid, called Boron siliconglass-BSG.

[0055] The boron diffusion doping process of the present invention (as shown in Table 1 of Example 1 below) introduces more oxygen than the conventional boron diffusion doping process (as shown in Table 2 of Comparative Example 1 below) during the pre-oxygen process to form a thicker oxide layer, which helps to block boron atoms from entering the silicon bulk region during subsequent deposition. Furthermore, by reducing the amount of boron source used during deposition, the overall boron doping concentration of the lightly doped structure after the boron diffusion doping process of the present invention is lower than that of the conventional boron diffusion doping process, reducing recombination losses. This helps to achieve extremely light doping within the silicon bulk region of the metallization area. For example, the oxygen flow rate during the pre-oxygen process is greater than or equal to 300 sccm (preferably 300 sccm), and the boron source flow rate during the deposition process is less than or equal to 250 sccm (preferably 250 sccm).

[0056] Then, through the laser scanning process using short-wavelength high-energy lasers in step 3, combined with the annealing process in step 5, the doping concentration of the lightly doped structure in the shallow surface layer of the silicon wafer's metallized region is increased, achieving heavy doping in the shallow surface layer to reduce metal-to-metal recombination. Furthermore, the synergistic effect of steps 1-3 and step 5 achieves the goal of reducing recombination in both the shallow surface layer of the silicon wafer's metallized region and the recombination within the silicon bulk region beneath it, thereby lowering contact resistance and improving contact performance.

[0057] Step 2: Clean the back side of the silicon wafer with hydrofluoric acid (HF) solution and polish the back side. In the final process of the back side polishing process, remove the BSG on the front side of the silicon wafer by hydrofluoric acid pickling to expose the lightly doped structure.

[0058] Step 3: Use a short-wave high-energy laser to scan the lightly doped structure of the shallow surface layer of the predetermined metallization region of the silicon wafer, so that the crystalline silicon in the lightly doped structure of the shallow surface layer becomes amorphous, while the non-metallization region of the silicon wafer retains the crystalline silicon structure.

[0059] In step 3, the laser scanning process conditions for the short-wavelength high-energy laser are as follows: the wavelength of the short-wavelength high-energy laser is less than or equal to 355nm (preferably a 355nm ultraviolet picosecond laser), and the laser energy density is 110-140mJ / cm². 2 (Preferred value: 131mJ / cm) 2The laser power is 0.50-0.60W (e.g., 0.58W), and the scanning time is very short (e.g., 0.1-0.2s). The shallow layer is the subsurface of the silicon wafer, located at a depth of tens of nanometers (e.g., 30-60nm) from the surface of the silicon wafer.

[0060] The texturing process on silicon wafers creates a cone-shaped inverted pyramid structure to increase light absorption by subjecting natural light to multiple reflections and refractions. Shorter wavelength ultraviolet light is rapidly absorbed and consumed in the shallow surface layer (at a depth of tens of nanometers from the silicon wafer surface). Therefore, the penetration depth of short-wavelength high-energy lasers is very shallow. Furthermore, during the laser scanning process, the crystalline silicon in the lightly doped structure of the silicon wafer subsurface transforms into amorphous silicon (a-Si). In other words, the silicon in the lightly doped structure of the shallow surface layer of the metallized region is in an amorphous silicon state. This is because the laser energy of the short-wavelength high-energy laser is rapidly absorbed by the crystalline silicon at the silicon wafer subsurface. The instantaneous high energy breaks the crystal bonds in the silicon lattice at the subsurface, causing it to change from a crystalline silicon structure to an amorphous silicon structure. Compared to crystalline silicon, the doping atoms activated by the combined laser scanning process in step 3 and the annealing process in step 5 have a higher doping concentration in the recrystallized crystalline silicon. This is because during the recrystallization of amorphous silicon into crystalline silicon, the impurity atoms can better combine with silicon atoms from their independent interstitial states to form stable, activated substitutional states.

[0061] Therefore, through the synergistic effect of the annealing process in steps 1-3 and the subsequent step 5, not only can a lightly doped structure be formed in the non-metallized region of the silicon wafer, and a heavily doped structure be formed in the shallow layer of the metallized region of the silicon wafer, but also an extremely low doping concentration can be achieved in the bulk region located below the shallow layer of the metallized region of the silicon wafer (the bulk region is located at a depth of more than 60 nm from the surface of the silicon wafer).

[0062] Step 4: Sequentially prepare a tunneling layer and a doped amorphous silicon layer (doping source is phosphorus source) on the back side of the silicon wafer. The specific process of step 4 is the same as that of existing TOPCon cell fabrication process, so it will not be described in detail here.

[0063] Step 5: Annealing, which converts amorphous silicon into crystalline silicon. During the recrystallization of amorphous silicon into crystalline silicon, doped atoms are activated, which increases the doping concentration of the shallow layer of the metallized region compared to the non-metallized region, so as to form a selective emitter on the front side of the silicon wafer and convert the doped amorphous silicon layer on the back side of the silicon wafer into a doped polycrystalline silicon layer.

[0064] In step 5, the annealing process conditions are as follows: annealing temperature is 900-950℃ (preferably 950℃), and constant temperature annealing time is 30-50min (preferably 30min).

[0065] In step 5, the main functions of the annealing process are: firstly, crystallization, which includes both the crystallization of the lightly doped amorphous silicon in the shallow layer of the metallized region on the front side of the silicon wafer and the crystallization of the doped amorphous silicon layer on the back side of the silicon wafer. Simultaneously, secondly, it activates dopant atoms that were originally inactive to increase the doping concentration and achieve heavy doping. This includes activating boron dopant atoms in the lightly doped structure of the shallow layer of the metallized region on the front side of the silicon wafer to achieve heavy doping, forming a heavily doped structure in the shallow layer of the metallized region while still ensuring an extremely low doping concentration in the bulk region below the shallow layer of the metallized region; and also activating phosphorus dopant atoms in the doped amorphous silicon layer to achieve heavy doping.

[0066] Steps 1-3 and 5 above constitute a method for selective heavy doping of silicon surfaces according to the present invention. While the conventional TOPCon cell fabrication process in Comparative Example 1 also requires annealing to convert the doped amorphous silicon layer into a doped polycrystalline silicon layer, applying the method of selective heavy doping of silicon surfaces of the present invention to the fabrication method of photovoltaic cells (such as TOPCon cells) eliminates the need for an additional annealing step. Of course, the method of selective heavy doping of silicon surfaces of the present invention can also be applied to the fabrication of other types of crystalline silicon solar cells.

[0067] Step 6: After annealing, the silicon wafer is sequentially cleaned, and passivation films are deposited on both the front and back sides. Laser-engraved passivation films are then applied to the front and back sides of the metallized area, followed by electroplating of front and back metal electrodes (such as copper electrodes). This process creates a TOPCon cell with a heavily doped shallow surface layer of the metallized area on the front side and a very lightly doped interior region beneath the shallow surface layer. The specific process of Step 6 is based on existing TOPCon cell fabrication processes and will not be elaborated here.

[0068] Example 1

[0069] This embodiment describes a method for selective heavy doping of silicon surfaces, a photovoltaic cell, and its fabrication method. Referring to steps 1-6 of the specific implementation method using a TOPCon cell as an example described above, the difference in this embodiment is:

[0070] The boron diffusion doping process conditions in step 1 of this embodiment are shown in Table 1 below:

[0071] Table 1. Process conditions for boron diffusion doping in Example 1 of the present invention.

[0072]

[0073] The laser scanning process conditions in step 3 of this embodiment are as follows: a 355nm wavelength ultraviolet picosecond laser is used, and the laser energy density is 131mJ / cm². 2 The laser power is 0.58W.

[0074] The annealing process in step 5 of this embodiment is as follows: the annealing temperature is 950℃ and the constant temperature annealing time is 30min.

[0075] Comparative Example 1

[0076] This comparative example provides a photovoltaic cell and its preparation method, referring to Example 1, but differing from Example 1 in that:

[0077] The process conditions for boron diffusion doping in step 1 of this comparative example are shown in Table 2 below (mainly: less oxygen was introduced during the pre-oxygen process than in Example 1, and more boron source was used during the deposition process than in Example 1):

[0078] Table 2. Process conditions for conventional boron diffusion doping in Comparative Example 1

[0079]

[0080] This comparative example also omits step 3 (i.e., short-wave high-energy laser scanning) of Example 1.

[0081] Performance testing

[0082] 1. Select a laser with an energy density of 110-140 mJ / cm². 2 After laser scanning processing of multiple energy density values, a TOPCon battery is obtained (the preparation steps of the battery are as described in Example 1). The open-circuit voltage of the TOPCon battery is tested. The open-circuit voltage (Voc) performance of the battery can be improved by optimizing laser parameters (such as laser energy density E), thereby improving the battery efficiency.

[0083] See Figure 3 As can be seen, because a light doping process was used in step 1 of this invention, the open-circuit voltage (Voc) of the battery was low in step 3 when the energy density (E) of the short-wavelength high-energy laser scanning treatment was low; as the energy density of the short-wavelength high-energy laser scanning treatment increased, the doping concentration of the lightly doped structure in the shallow layer increased rapidly, and therefore the open-circuit voltage also increased accordingly. When the energy density increased to 131 mJ / cm², the open-circuit voltage increased further. 2 The highest open-circuit voltage is at a time (as in Example 1).

[0084] like Figure 3 As shown, when the energy density of the short-wavelength high-energy laser continues to increase, it introduces laser damage along with the increased doping concentration. Although some laser damage can be repaired through wet processing and high-temperature processing, there are limits to the repair, making it difficult to completely eliminate the laser damage. Consequently, the open-circuit voltage of the battery begins to gradually decrease. After comprehensive consideration, the optimal energy density of this short-wavelength high-energy laser is 131 mJ / cm². 2 .

[0085] Furthermore, regarding the selection of the laser wavelength in step 3, since this laser only needs to act on the lightly doped structure in the shallow surface layer of the silicon wafer's metallization region, and longer wavelength lasers have strong light transmission and will not be completely absorbed in the shallow surface layer, short-wavelength, high-energy ultraviolet light is chosen for step 3. Common ultraviolet laser wavelengths are generally 266nm and 355nm, with the 266nm laser's effective range being only a few or a dozen nanometers. In the subsequent metal electrode fabrication step 6, the metal (such as copper) will also erode a portion of the silicon substrate (this will destroy the heavily doped structure at a depth of a few or a dozen nanometers below the silicon wafer's surface). Therefore, when the laser's effective depth is shallow (e.g., only a few or a dozen nanometers), the metal electrode will come into contact with the lightly doped structure, leading to a decrease in battery performance, including contact resistance.

[0086] Therefore, in step 3, the laser wavelength is considered in relation to the depth of the doped atoms, and the energy density is considered in relation to the enhancement of open-circuit voltage and the final improvement of battery performance. Thus, the laser scanning process in step 3 preferably uses an ultraviolet laser with a wavelength of 355 nm and an energy density of 131 mJ / cm². 2 .

[0087] 2. Performance tests were conducted on the boron diffusion doping concentration-doping depth of the metallized region on the front side of the batteries in Example 1 and Comparative Example 1, respectively, and the results were as follows: Figure 4-5 The curves shown represent the boron doping concentration versus doping depth.

[0088] pass Figure 4 As can be seen, in Example 1, the boron doping concentration reaches its maximum value of 1.30E19cm at 0.032µm (i.e., at a depth of 32nm from the front surface of the battery). -3 In Example 1, the boron doping concentration had decreased to 4.50E18cm at 0.4µm. -3 Compared to the conventional boron diffusion doping process in Comparative Example 1, the boron doping concentration at 0.4 μm (e.g., ...) Figure 5 In Example 1, the boron doping concentration was reduced by more than three times, thus Example 1 achieved extremely light doping in the silicon bulk region located below the shallow layer of the metallized region.

[0089] 3. Battery performance (e.g., composite density J0, battery efficiency Eta) was tested on Example 1 and Comparative Example 1 respectively, and the results were as follows: Figure 6 The composite density J0 test data shown, and as follows Figure 7 The battery efficiency Eta test data shown is presented.

[0090] See Figure 6Comparing the composite density of Comparative Example 1 (which uses a conventional TOPCon cell fabrication process, but with a different boron doping diffusion in step 1 and no short-wavelength high-energy laser scanning) and Example 1 (which uses a light doping process in step 1, and includes short-wavelength high-energy laser scanning in step 3 after removing the borosilicate glass in step 2 and before annealing in step 5), the composite density J0 of Comparative Example 1 reaches 4.08 fA / cm². 2 The composite density of the battery in Example 1 decreased to 2.89 fA / cm². 2 The decrease reached 29.2%. (See also...) Figure 7 By comparing the final battery efficiency, it can be found that due to the smaller composite size in Example 1, Example 1 can improve the battery efficiency by about 0.11% compared to the conventional TOPCon battery fabrication process in Comparative Example 1.

[0091] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the present invention.

[0092] The technical solution provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for selective heavy doping of silicon surfaces, characterized in that, Includes the following steps: S1. A light doping process is used to diffusely dope the surface of a silicon wafer to sequentially form a lightly doped structure and doped silicon glass on the silicon wafer; the silicon wafer is crystalline silicon. S2. Remove the doped silicon glass to expose the lightly doped structure; S3. A short-wavelength high-energy laser is used to scan the lightly doped structure of the shallow surface layer of the metallized region of the silicon wafer, so that the crystalline silicon in the lightly doped structure of the shallow surface layer becomes amorphous, while the non-metallized region of the silicon wafer retains the crystalline silicon structure. The short-wavelength high-energy laser has a wavelength less than or equal to 355 nm and an energy density of 110-140 mJ / cm². 2 ; S4. Annealing transforms amorphous silicon into crystalline silicon. During the recrystallization of amorphous silicon into crystalline silicon, doped atoms are activated, increasing the doping concentration of the shallow layer of the metallized region compared to the non-metallized region, thus producing a selective emitter.

2. The method for selective heavy doping of silicon surface according to claim 1, characterized in that, In step S1, boron diffusion doping is performed on the front side of the silicon wafer; the temperature of the light doping process is 800-900℃, the time is 40-70min, the doping source is a boron source, the boron source flow rate is 200-250sccm, and the oxygen flow rate is 300-450sccm.

3. The method for selective heavy doping of silicon surface according to claim 2, characterized in that, In step S1, the light doping process includes a pre-oxygen process, a deposition process, a propulsion process, and a post-oxygen process performed sequentially; the oxygen flow rate in the pre-oxygen process is greater than or equal to 300 sccm.

4. The method for selective heavy doping of silicon surface according to claim 3, characterized in that, In step S1, the boron source flow rate during the deposition process is less than or equal to 250 sccm.

5. The method for selective heavy doping of silicon surface according to claim 1, characterized in that, In step S3, the laser power of the short-wave high-energy laser is 0.50-0.60W; the shallow layer is the subsurface of the silicon wafer, located at a depth of 30-60nm from the surface of the silicon wafer.

6. The method for selective heavy doping of silicon surface according to claim 5, characterized in that, In step S3, the short-wavelength high-energy laser is a 355nm ultraviolet picosecond laser with a laser power of 0.58W and a laser energy density of 131mJ / cm². 2 .

7. The method for selective heavy doping of silicon surface according to claim 1, characterized in that, In step S4, the annealing temperature is 900-950℃ and the time is 30-50 minutes; After annealing, the peak doping concentration in the shallow layer of the metallized region is 1.20E19cm. -3 -1.50E19cm -3 The doping concentration within the silicon bulk region beneath the shallow layer of the metallized region rapidly decreases to zero at a depth of 0.6-0.8 μm, while the peak doping concentration of the lightly doped structure in the non-metallized region is 1.00E19 cm⁻¹. -3 -1.45E19cm -3 .

8. A method for preparing a photovoltaic cell, characterized in that, It includes a method for selective heavy doping of a silicon surface as described in any one of claims 1-7; the photovoltaic cell is a crystalline silicon solar cell.

9. A method for preparing a photovoltaic cell according to claim 8, characterized in that, The photovoltaic cell is a TOPCon cell, which includes the following preparation steps: Step 1: Using a light doping process, diffusion doping is performed on one surface of the texturized silicon wafer to sequentially form a lightly doped structure and doped silicon glass on the silicon wafer. The silicon wafer is crystalline silicon; Step 2: Perform single-sided acid cleaning and polishing on the other surface of the silicon wafer, and at the end of the polishing process, use double-sided acid cleaning to remove the doped silicon glass to expose the lightly doped structure. Step 3: Scan the lightly doped structure of the shallow surface layer of the metallized region of the silicon wafer using a short-wavelength high-energy laser to amorphize the crystalline silicon in the lightly doped structure of the shallow surface layer, while the non-metallized region of the silicon wafer retains the crystalline silicon structure; the wavelength of the short-wavelength high-energy laser is less than or equal to 355 nm, and its energy density is 110-140 mJ / cm². 2 ; Step 4: Sequentially prepare a tunneling layer and a doped amorphous silicon layer on the other surface of the silicon wafer; Step 5: Annealing, which converts amorphous silicon into crystalline silicon. During the recrystallization of amorphous silicon into crystalline silicon, doping atoms are activated, which increases the doping concentration of the shallow layer of the metallized region compared to the non-metallized region, so as to form a selective emitter on one surface of the silicon wafer and convert the doped amorphous silicon layer on the other surface of the silicon wafer into a doped polycrystalline silicon layer. Step 6: After annealing, the silicon wafer is sequentially cleaned, and passivation films on the front and back sides are deposited. Then, passivation films on the front and back sides of the laser-opened metallized areas are deposited, and metal electrodes on the front and back sides are electroplated to obtain the TOPCon cell.

10. A photovoltaic cell, characterized in that, It is prepared by the method of photovoltaic cell preparation described in claim 8 or 9.

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