Preparation method of selective emitter and solar cell

Abstract

The present application discloses a preparation method of a selective emitter and a solar cell including the selective emitter. The preparation method sequentially includes the following steps: diffusing doping elements on the surface of a silicon substrate to form a diffusion layer and a silicon oxide layer from the inside to the outside on the surface of the silicon substrate; using a laser to groove partial regions on the surface of the silicon oxide layer on one side of the silicon substrate, and enabling the bottom of the groove to reach the diffusion layer; removing porous silicon in the laser grooved region and retaining at least part of the laser damage; and performing secondary diffusion of the doping elements on the diffusion layer in the laser grooved region, so that the diffusion layer includes a lightly doped region with a first doping concentration and a heavily doped region with a second doping concentration. The selective emitter can improve the contact performance between the metal grid electrode and the silicon substrate, and further improve the conversion efficiency of the corresponding solar cell.

Description

Technical Field

[0001] The present application relates to the field of solar cells, and more particularly, to a method for preparing a selective emitter and a solar cell including the selective emitter. Background Art

[0002] A solar cell is a photoelectric conversion device that directly generates electricity using sunlight. As long as it receives light under certain illumination conditions, it can instantaneously output a voltage and generate a current when there is a circuit. Specifically, a solar cell chip includes a semiconductor p-n junction. When illuminated, new hole-electron pairs are formed. Under the action of the built-in electric field of the p-n junction, the photo-generated holes flow to the p region, and the photo-generated electrons flow to the n region. After connecting an electric circuit, a current can be generated. This is the photoelectric effect and also the working principle of the solar cell.

[0003] In solar cells, the initially adopted was a uniform emitter. However, the uniform emitter has the following defects: when the sheet resistance of the semiconductor in the emitter region is high, the emitter recombination is low, while the recombination of the metal grid electrode is high; when the sheet resistance of the emitter region is low, the recombination of the metal grid electrode is low, while the emitter recombination is high. This will affect the conversion efficiency of the solar cell. In response to this, a selective emitter solar cell has been further developed, that is, heavy doping is performed at the position of the semiconductor substrate where the metal grid electrode is located, and light doping is performed at the position between the electrodes to optimize the emitter region. Thereby, the recombination of hole-electron pairs in the diffusion layer can be reduced, the contact resistance between the electrode and the substrate can be reduced, and at the same time, both the emitter recombination and the recombination of the metal grid electrode can be kept at a low level, thereby improving the conversion efficiency of the solar cell.

[0004] However, for selective emitter solar cells, how to further improve the selective emitter preparation process to further increase the conversion efficiency of solar cells is of great significance. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present application provides a method for preparing a selective emitter and a solar cell including the selective emitter. In the method for preparing the selective emitter of the present application, surface damage and defects are created by laser grooving at the position of the silicon substrate where the metal grid electrode is to be provided, then the degree of laser damage is adjusted, and secondary doping is performed at the grooved position, thereby obtaining the selective emitter according to the present application. The selective emitter can improve the contact performance between the metal grid electrode and the silicon substrate, reduce the contact resistance, and further improve the conversion efficiency of the solar cell including the selective emitter.

[0006] To achieve the above object, in a first aspect, the present application provides a method for preparing a selective emitter, which successively includes the following steps:

[0007] Diffuse doping elements on the surface of the silicon substrate to form a diffusion layer and a silicon oxide layer from the inside to the outside on the surface of the silicon substrate, wherein the diffusion layer includes doping elements with a first doping concentration;

[0008] Use a laser to groove a part of the surface of the silicon oxide layer on one side of the silicon substrate, and make the bottom of the groove reach the diffusion layer;

[0009] Remove the porous silicon in the laser grooved area and retain at least part of the laser damage; and

[0010] Perform secondary diffusion of the doping elements on the diffusion layer in the laser grooved area, so that the diffusion layer includes a lightly doped region with a first doping concentration and a heavily doped region with a second doping concentration, wherein the heavily doped region corresponds to the laser grooved area, and the second doping concentration is greater than the first doping concentration.

[0011] Combined with the first aspect, in a feasible implementation manner, the removing the porous silicon in the laser grooved area and retaining at least part of the laser damage includes the following steps: etching the laser grooved area for 1 min to 3 min with a solution containing 1% to 5% by mass of KOH and 3% to 5% by mass of H2O2 under a temperature condition of 60 °C to 80 °C.

[0012] Combined with the first aspect, in another feasible implementation manner, the removing the porous silicon in the laser grooved area and retaining at least part of the laser damage includes the following steps: oxidizing the laser grooved area for 30 min to 60 min under a temperature of 800 °C to 900 °C and an oxygen atmosphere, and then cleaning with hydrofluoric acid with a mass concentration of 2% to 8% for 3 min to 6 min.

[0013] Combined with the first aspect, in yet another feasible implementation manner, the removing the porous silicon in the laser grooved area and retaining at least part of the laser damage includes the following steps: irradiating the laser grooved area with a plasma at an output power of 1000 W to 1500 W for 5 min to 10 min.

[0014] Combined with the first aspect, in a feasible implementation manner, when the silicon substrate is an N-type silicon substrate, the diffusion layer is a P+ doping layer. Further, in a feasible implementation manner, the doping element is at least one of boron, aluminum, gallium or indium, preferably boron.

[0015] Further, under the above P-type doping conditions, the sheet resistance of the diffusion layer is 150 Ω / sqr to 250 Ω / sqr; the diffusion thickness of the diffusion layer is 0.4 μm to 0.8 μm; and the thickness of the silicon oxide layer is 0.07 μm to 0.11 μm.

[0016] Further, under the above P-type doping conditions, the sheet resistance of the lightly doped region is 150 Ω / sqr to 250 Ω / sqr, the sheet resistance of the heavily doped region is 10 Ω / sqr to 100 Ω / sqr; and the thickness of the heavily doped region is 0.7 μm to 1.2 μm.

[0017] Combined with the first aspect, in a feasible implementation manner, when the silicon substrate is a P-type silicon substrate, the diffusion layer is an N+ doped layer. Further, in a feasible implementation manner, the doping element is at least one of phosphorus, arsenic or antimony, preferably phosphorus.

[0018] Further, under the above N-type doping conditions, the sheet resistance of the diffusion layer is 150 Ω / sqr to 250 Ω / sqr; the diffusion thickness of the diffusion layer is 0.2 μm to 0.4 μm; and the thickness of the silicon oxide layer is 0.04 μm to 0.08 μm.

[0019] Further, under the above N-type doping conditions, the sheet resistance of the lightly doped region is 150 Ω / sqr to 250 Ω / sqr, the sheet resistance of the heavily doped region is 10 Ω / sqr to 100 Ω / sqr; and the thickness of the heavily doped region is 0.4 μm to 1.0 μm.

[0020] Combined with the first aspect, in a feasible implementation manner, the diffusion in step (1) and the secondary diffusion in step (4) are each independently carried out by any one or more of a high-temperature diffusion process, a paste doping process or an ion implantation process.

[0021] In a second aspect, the present application provides a solar cell, which is prepared by a method sequentially including the following steps:

[0022] Texturize the surface of the silicon substrate;

[0023] Prepare a selective emitter on the front surface of the silicon substrate according to the preparation method described in the first aspect above;

[0024] Etch the front and back surfaces of the silicon substrate to remove the silicon oxide layer;

[0025] Perform single-sided etching on the back surface of the silicon substrate to remove the diffusion layer;

[0026] Deposit a passivation layer and / or an antireflection layer on the surface of the silicon substrate; and

[0027] Perform metallization treatment on the surface of the selective emitter and the back surface of the silicon substrate to obtain a front electrode and a back electrode, and the front electrode and the back electrode respectively penetrate the antireflection layer and / or the passivation layer to form an ohmic contact with the silicon substrate.

[0028] In a third aspect, the present application provides a solar cell obtained by the preparation method adopted in the second aspect above, and includes a front electrode (metal grid electrode), a front antireflection layer and / or passivation layer, a selective emitter prepared by the preparation method according to the first aspect above, a silicon substrate, a back passivation layer, and a back electrode, which are arranged in sequence from top to bottom.

[0029] In a fourth aspect, the present application provides a photovoltaic module, which includes glass, encapsulation material, at least one solar cell according to the second or third aspect of the present application, encapsulation material, and a backplane, which are arranged in sequence from top to bottom.

[0030] The technical solution provided by the present application has at least the following beneficial effects compared with the prior art:

[0031] According to the preparation method of the selective emitter of the present application, damage and defects are created by laser grooving, then the porous silicon damage left in the grooves is removed and at least part of the laser damage below the porous silicon is retained, and finally a heavily doped region is formed at the grooving position, thereby obtaining the selective emitter according to the present application. The selective emitter can improve the contact performance between the metal grid electrode and the silicon substrate, reduce the contact resistance, and thus improve the conversion efficiency of the solar cell including the selective emitter.

[0032] In the preparation process of the selective emitter of the present application, a low-cost simplified process scheme that can be mass-produced is adopted. The preparation procedure is simple, the operation is convenient, and the efficiency is high. In particular, the selective emitter is formed by a method of two doping steps plus laser grooving. Surface defects are deliberately formed in the heavily doped region, and a certain amount of laser damage is retained to reduce the contact resistance between the metal and the semiconductor, thereby improving the performance of the selective emitter.

[0033] The solar cell including the above selective emitter according to the present application has a high conversion efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic flow chart of the preparation method of the selective emitter according to an embodiment of the present application;

[0035] Figure 2 is a scanning electron microscope (SEM) image of a part of the bottom of a certain groove of a silicon substrate after laser grooving but before removing the porous silicon according to Example 1 of the present application;

[0036] Figure 3 is a scanning electron microscope (SEM) image of another part of the bottom of the groove of the silicon substrate after laser grooving but before removing the porous silicon according to Example 1 of the present application;

[0037] Figure 4It is a scanning electron microscope (SEM) image of a certain part of the bottom of a silicon substrate after laser grooving and removal of porous silicon according to Embodiment 1 of the present application;

[0038] Figure 5 It is a scanning electron microscope (SEM) image of another part of the bottom of a silicon substrate after laser grooving and removal of porous silicon according to Embodiment 1 of the present application. Description of the Drawings

[0040] 1. Silicon substrate; 2. Diffusion layer; 3. Silicon oxide layer; 4. Laser grooving area; 41. Porous silicon; 42. Laser damage; 5. Lightly doped region; 6. Heavily doped region. Detailed Embodiments

[0041] In order to enable those skilled in the art to understand the present application more clearly, the present application will be further described in detail below in conjunction with embodiments and drawings. However, it should be understood that the following embodiments are only the preferred embodiments of the present application, and the scope claimed by the present application should be subject to the scope defined by the claims.

[0042] It should be understood that in the present application, the direction terms such as "front", "up", and "positive" refer to the direction corresponding to the side of the solar cell or the silicon substrate to be made into a solar cell that faces or will face the light source; correspondingly, the direction terms such as "back", "down", and "rear" refer to the direction corresponding to the side of the solar cell or the silicon substrate to be made into a solar cell that backs or will back the light source.

[0043] It should be understood that in the present application, when it is mentioned that a component is "above" or "below" another component or similar descriptions, such descriptions not only mean that the component is in direct contact with and is "above" or "below" the other component, but also mean that the component is not in direct contact with and is "above" or "below" the other component, that is, there may be a space and / or other components between the two.

[0044] It should be understood that in the present application, the unit of the sheet resistance can be expressed as "ohm / sq", "ohm / sqr", "Ω / sq" or "Ω / sqr", which all represent the same meaning and the same unit measure and can be used interchangeably.

[0045] In a first aspect, the present application provides a method for preparing a selective emitter, which sequentially includes the following steps:

[0046] Diffuse doping elements on the surface of the silicon substrate to form a diffusion layer and a silicon oxide layer from the inside to the outside on the surface of the silicon substrate, wherein the diffusion layer includes doping elements with a first doping concentration;

[0047] Use a laser to groove partial regions on the surface of the silicon dioxide layer on one side of the silicon substrate, and make the bottom of the groove reach the diffusion layer;

[0048] Remove the porous silicon in the laser grooving region and retain at least part of the laser damage; and

[0049] Perform secondary diffusion of the doping element on the diffusion layer in the laser grooving region, so that the diffusion layer includes a lightly doped region with a first doping concentration and a heavily doped region with a second doping concentration, where the heavily doped region corresponds to the laser grooving region, and the second doping concentration is greater than the first doping concentration.

[0050] In this application, the preparation method of the selective emitter according to this application first performs element doping on the surface of the silicon substrate to form a diffusion layer and a silicon dioxide layer, then the laser grooving penetrates the silicon dioxide layer to create surface damage and defects on the diffusion layer, then adjusts the degree of laser damage (that is, removes the porous silicon structure formed in the groove after laser grooving, but retains at least part of the laser burning damage on the surface below the porous silicon structure), and finally performs secondary element doping (forms a heavily doped region) at the grooving position, thereby obtaining the selective emitter according to this application. The selective emitter can improve the contact performance between the metal gate line electrode and the silicon substrate, reduce the contact resistance, and thus improve the conversion efficiency of the solar cell including the selective emitter.

[0051] In this application, the silicon substrate can be any one of a polysilicon substrate, a monocrystalline silicon substrate, or a quasi-monocrystalline silicon substrate, all of which are applicable to the technical solution of this application. Therefore, in the embodiments of this application, the specific type of the silicon substrate is not particularly limited.

[0052] In the step of diffusing the doping element on the surface of the silicon substrate, the diffusion can be carried out by any one or more of a high-temperature diffusion process, a paste doping process, or an ion implantation process, all of which are applicable to the technical solution of this application, and preferably a high-temperature diffusion process, which is simple in operation and low in cost. In addition, the diffusion of the doping element in this application is carried out at least on one side of the silicon substrate to facilitate the preparation of the selective emitter, and this side can be the front side facing the sunlight. Whether the back side of the silicon substrate opposite to the front side simultaneously undergoes the above diffusion is not particularly limited in this application, but usually the back side is not masked, that is, the two sides of the silicon substrate are diffused simultaneously, which can reduce the masking process and cost, and can also remove the diffusion layer and silicon dioxide layer on the back side and the silicon dioxide layer on the front side through etching in the subsequent preparation process of the solar cell. Therefore, the two-sided diffusion will not affect the performance of the solar cell.

[0053] The silicon oxide layer is formed on the outside while forming a diffusion layer on the surface of the silicon substrate. The silicon oxide layer does not need to be removed during the preparation of the selective emitter and has the following uses: The silicon oxide layer can be used as a barrier layer to prevent the corresponding doping elements from depositing on the diffusion layer area covered by the silicon oxide layer during the secondary diffusion process in step (4), thereby ensuring that the area covered by the silicon oxide layer maintains a relatively high sheet resistance and forming a lightly doped region.

[0054] In this application, the silicon substrate can be a P-type silicon substrate or an N-type silicon substrate, both of which are applicable to the technical solution of this application. Therefore, the specific type of the silicon substrate is not limited in the embodiments of this application.

[0055] Optionally, when the silicon substrate is an N-type silicon substrate, it can be P-type doped to form a P+ doped layer (diffusion layer). The doping element can be at least one of P-type elements such as boron, aluminum, gallium, or indium, preferably boron.

[0056] Under the above P-type doping conditions, the sheet resistance of the obtained diffusion layer can be 150 Ω / sqr to 250 Ω / sqr. For example, it can be 150 Ω / sqr, 160 Ω / sqr, 170 Ω / sqr, 180 Ω / sqr, 190 Ω / sqr, 200 Ω / sqr, 210 Ω / sqr, 220 Ω / sqr, 230 Ω / sqr, 240 Ω / sqr, 250 Ω / sqr, or other specific values within the range; the obtained diffusion thickness can be 0.4 μm to 0.8 μm. For example, it can be 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, 0.8 μm, or other specific values within the range; and the thickness of the obtained silicon oxide layer can be 0.07 μm to 0.11 μm. For example, it can be 0.07 μm, 0.075 μm, 0.08 μm, 0.085 μm, 0.09 μm, 0.095 μm, 0.10 μm, 0.105 μm, 0.11 μm, or other specific values within the range.

[0057] Optionally, when the silicon substrate is a P-type silicon substrate, it is N-type doped to form an N+ doped layer (diffusion layer). The doping element can be at least one of N-type elements such as phosphorus, arsenic, or antimony, preferably phosphorus.

[0058] Under the above N-type doping conditions, the sheet resistance of the obtained diffusion layer can be 150 Ω / sqr to 250 Ω / sqr. For example, it can be 150 Ω / sqr, 160 Ω / sqr, 170 Ω / sqr, 180 Ω / sqr, 190 Ω / sqr, 200 Ω / sqr, 210 Ω / sqr, 220 Ω / sqr, 230 Ω / sqr, 240 Ω / sqr, 250 Ω / sqr, or other specific values within the above range; the obtained diffusion thickness can be 0.2 μm to 0.4 μm. For example, it can be 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, or other specific values within the above range; and the thickness of the obtained silicon oxide layer can be 0.04 μm to 0.08 μm. For example, it can be 0.04 μm, 0.045 μm, 0.05 μm, 0.055 μm, 0.06 μm, 0.065 μm, 0.07 μm, 0.075 μm, 0.08 μm, or other specific values within the above range.

[0059] By appropriately controlling the first doping concentration of the doping element for P-type doping or N-type doping, the diffusion layer can have the above-mentioned sheet resistance. In addition, through doping of the opposite type to the silicon substrate, a PN junction structure can be formed between the formed diffusion layer and the silicon substrate to generate a photovoltaic effect.

[0060] Next, a laser is used to groove a part of the surface of the silicon oxide layer on one side of the silicon substrate, and the bottom of the groove reaches the silicon substrate where the diffusion layer is located. Among them, the grooved area is the position where the selective emitter is to be formed and also the position where the metal gate line electrode is to be provided. The energy of the laser is very concentrated, and it can generate extremely high temperature in the irradiated area, causing the substances in this area to directly vaporize and volatilize, thus generating a groove. Through laser irradiation and burning, the uppermost silicon oxide layer is burned through until the bottom of the groove exposes the diffusion layer. If there is still an oxide layer at the bottom of the groove, the oxide layer will have an adverse effect on the secondary diffusion of the doping element after this step, and it is not necessary to continue burning after the bottom of the groove exposes the diffusion layer, which will waste energy, and the diffusion layer that is too thin or even completely removed at this place is also not conducive to the secondary diffusion.

[0061] Next, the porous silicon in the laser grooving area is removed while at least part of the laser damage is retained. Since laser grooving is achieved through the thermal volatilization of the irradiated material, silicon with a porous structure (i.e., porous silicon) that fails to be completely volatilized or re-condenses after volatilization will remain on both the bottom and sidewalls of the groove. This structure is porous and fluffy, with a large surface area, and accumulates on the outermost sides of the bottom and sidewalls of the groove, covering the silicon substrate (laser damage) that is pitted but still dense in structure after being burned on its inner side. Although those skilled in the art generally believe that surface damage defects in the heavily doped region can improve the contact performance between the metal electrode and the semiconductor substrate, however, through research, it has been found that the surface of the substrate with this porous silicon structure is not easily passivated due to its overly fluffy structure. Although the contact area with the metal electrode deposited on this surface is increased, the porous structure instead results in a relatively large recombination current, which has an adverse effect on the improvement of the conversion efficiency of the solar cell. Therefore, by removing the above-mentioned porous silicon structure while still retaining a part of the pitted and dense laser-burned damaged silicon substrate, it is possible to increase the contact area between the metal electrode and the semiconductor, reduce the contact resistance, and at the same time reduce the recombination current with the metal electrode, thereby further improving the conversion efficiency of the solar cell.

[0062] As a feasible implementation, the porous silicon structures existing on the bottom and sidewalls of the groove can be removed while at least part of the laser damage after removal is retained; in another implementation of this application, the sidewalls of the groove can be masked with a masking agent in advance, and then only the porous silicon structure existing on the bottom of the groove is removed to expose at least part of the laser damage under the porous silicon at the bottom of the groove, and then the masking agent is removed, whereby only the porous silicon at the bottom can be removed while the porous silicon on the sidewalls of the groove is retained. Those skilled in the art can select the part where the porous silicon is removed according to their needs.

[0063] As a feasible implementation manner, removing the porous silicon in the laser grooving area and retaining at least part of the laser damage may include the following steps: etching the laser grooving area with a solution containing 1% - 5% by mass of KOH and 3% - 5% by mass of H2O2 at a temperature of 60°C - 80°C for 1 min - 3 min. Among them, the concentrations of the selected KOH and H2O2 are both relatively low, making the overall oxidizing property of the solution moderate. The porous silicon structure is fluffy and has a large surface area, so its reaction activity is strong and it is easily oxidized and etched. In contrast, the silicon substrate has a relatively small surface area, a dense structure, and its reaction activity and rate are much lower than those of the porous silicon, and it is covered by the porous silicon, so it is not easily etched. Therefore, there is a large etching difference between the porous silicon and the silicon substrate, which is beneficial to the removal of the porous silicon on the sidewall and / or bottom of the groove and the retention of the laser burning damage marks. In addition, the etching can be selectively carried out only on the laser grooving area by means of regional etching, or the entire silicon substrate including the laser grooving area can be immersed and etched. The part of the silicon substrate that has not been laser grooved is not easily oxidized and etched because it is covered with a silicon oxide layer and has a similarly dense structure.

[0064] As another feasible implementation manner, removing the porous silicon in the laser grooving area and retaining at least part of the laser damage may include the following steps: oxidizing the laser grooving area at a temperature of 800°C - 900°C in an oxygen atmosphere for 30 min - 60 min, and then cleaning it with hydrofluoric acid with a mass concentration of 2% - 8% for 3 min - 6 min. Due to the huge difference in the relative surface area between the porous silicon on the sidewall and / or bottom of the groove and the silicon substrate at the laser burning damage marks, the reaction activities are also significantly different. Moreover, the porous silicon is on the surface layer and covers the silicon substrate at the laser burning damage marks. Therefore, oxygen will first oxidize most of the porous silicon, which is then removed by hydrofluoric acid cleaning. The degree of oxidation of the silicon substrate is relatively small, and the generated silicon oxide will also be completely removed in the subsequent hydrofluoric acid cleaning.

[0065] In addition, since the part of the silicon substrate that has not been laser grooved is covered with a silicon oxide layer itself and the silicon oxide layer will be strengthened during the oxidation process, and the concentration of hydrofluoric acid is relatively low and the cleaning time is short, part of the silicon oxide layer in the non-laser grooving area can still be retained, which will not affect its function. For example, the silicon oxide layer formed during P-type doping can retain a thickness of 0.06 μm - 0.09 μm after being cleaned with hydrofluoric acid, and the silicon oxide layer formed during N-type doping can retain a thickness of 0.03 μm - 0.06 μm after being cleaned with hydrofluoric acid.

[0066] In a feasible implementation manner, removing the porous silicon within the laser grooving region and retaining at least part of the laser damage may include the following steps: irradiating the laser grooving region with a plasma at an output power of 1000 W to 1500 W for 5 min to 10 min. The low output power of the above plasma results in a low plasma concentration and low energy, thereby irradiating the laser grooving region or irradiating the entire surface of the silicon substrate including the laser grooving region. Since the porous silicon has a loose structure and is on the surface layer, it is easy to react and be removed. The laser burn damage is covered by the porous silicon structure, and the un-grooved region of the silicon substrate is covered with a dense silicon oxide layer, so obvious bombardment damage will not be caused, and the irradiated silicon substrate does not need to be cleaned. In addition, the gas source of the plasma may not be particularly limited as long as it can be ionized to generate plasma under appropriate conditions. For example, inert gases such as helium, neon, and argon, or other suitable gases such as nitrogen and ammonia can be used.

[0067] In the present application, the second diffusion is carried out using the same type of doping element as the diffusion, that is, when the diffusion is P-type doping, the second diffusion is also P-type doping; when the diffusion is N-type doping, the second diffusion is also N-type doping. Through the second diffusion, the concentration of the doping element (the second doping concentration) can be further increased on the basis of the doping (the first doping concentration) at the diffusion layer within the laser grooving region, and the sheet resistance of this region can be reduced, thereby forming a heavily doped region. The region where the second diffusion is not carried out (that is, the diffusion layer part between the laser grooving regions) has a higher sheet resistance because its doping concentration is lower than that of the heavily doped region, so it becomes a lightly doped region. Thus, the selective emitter according to the present application can be formed.

[0068] Furthermore, under the condition that the secondary diffusion is P-type doping, the sheet resistance of the obtained lightly doped region is also in the range of 150 Ω / sqr to 250 Ω / sqr. For example, it can be 150 Ω / sqr, 160 Ω / sqr, 170 Ω / sqr, 180 Ω / sqr, 190 Ω / sqr, 200 Ω / sqr, 210 Ω / sqr, 220 Ω / sqr, 230 Ω / sqr, 240 Ω / sqr, 250 Ω / sqr, or other specific values within the range; the thickness of the obtained lightly doped region is also in the range of 0.4 μm to 0.8 μm. For example, it can be 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, 0.8 μm, or other specific values within the range; the sheet resistance of the obtained heavily doped region can be 10 Ω / sqr to 100 Ω / sqr. For example, it can be 10 Ω / sqr, 15 Ω / sqr, 20 Ω / sqr, 25 Ω / sqr, 30 Ω / sqr, 35 Ω / sqr, 40 Ω / sqr, 45 Ω / sqr, 50 Ω / sqr, 55 Ω / sqr, 60 Ω / sqr, 65 Ω / sqr, 70 Ω / sqr, 75 Ω / sqr, 80 Ω / sqr, 85 Ω / sqr, 90 Ω / sqr, 95 Ω / sqr, 100 Ω / sqr, or other specific values within the range; and the thickness of the obtained heavily doped region can be 0.7 μm to 1.2 μm. For example, it can be 0.7 μm, 0.75 μm, 0.8 μm, 0.85 μm, 0.9 μm, 0.95 μm, 1.0 μm, 1.05 μm, 1.1 μm, 1.15 μm, 1.2 μm, or other specific values within the range. Due to the influence of the secondary diffusion, relatively, the sheet resistance and thickness of the obtained lightly doped region may be greater than or equal to the sheet resistance and thickness of the corresponding diffusion layer.

[0069] Further, under the condition that the secondary diffusion is N-type doping, the sheet resistance of the obtained lightly doped region is also in the range of 150 Ω / sqr to 250 Ω / sqr. For example, it can be 150 Ω / sqr, 160 Ω / sqr, 170 Ω / sqr, 180 Ω / sqr, 190 Ω / sqr, 200 Ω / sqr, 210 Ω / sqr, 220 Ω / sqr, 230 Ω / sqr, 240 Ω / sqr, 250 Ω / sqr, or other specific values within the range; the thickness of the obtained lightly doped region is also in the range of 0.2 μm to 0.4 μm. For example, it can be 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, or other specific values within the range; the sheet resistance of the obtained heavily doped region can be 10 Ω / sqr to 100 Ω / sqr. For example, it can be 10 Ω / sqr, 15 Ω / sqr, 20 Ω / sqr, 25 Ω / sqr, 30 Ω / sqr, 35 Ω / sqr, 40 Ω / sqr, 45 Ω / sqr, 50 Ω / sqr, 55 Ω / sqr, 60 Ω / sqr, 65 Ω / sqr, 70 Ω / sqr, 75 Ω / sqr, 80 Ω / sqr, 85 Ω / sqr, 90 Ω / sqr, 95 Ω / sqr, 100 Ω / sqr, or other specific values within the range; and the thickness of the obtained heavily doped region can be 0.4 μm to 1.0 μm. For example, it can be 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm, 0.8 μm, 0.85 μm, 0.9 μm, 0.95 μm, 1.0 μm, or other specific values within the range. Due to the influence of the secondary diffusion, relatively, the sheet resistance and thickness of the obtained lightly doped region may be greater than or equal to the sheet resistance and thickness of the corresponding diffusion layer.

[0070] In addition, the secondary diffusion can be carried out by any one or more of a high-temperature diffusion process, a slurry doping process, or an ion implantation process, all of which are applicable to the technical solution of this application, and preferably a high-temperature diffusion process, which is simple to operate and low in cost.

[0071] With the suitable sheet resistance and thickness of the heavily doped region and the lightly doped region described above, it helps to form a good ohmic contact between the heavily doped region and the metal electrode, reduce the contact resistance, and enable the lightly doped layer to fully absorb short-wavelength light, increase the short-circuit current, thereby improving the photoelectric conversion efficiency of the solar cell and enhancing the performance of the solar cell.

[0072] In addition, the present application also provides a selective emitter, which is prepared by the preparation method described in the first aspect above. The selective emitter can improve the contact performance between the metal gate line electrode and the silicon substrate, reduce the contact resistance, and thus improve the conversion efficiency of the solar cell including the selective emitter.

[0073] In a second aspect, the present application provides a solar cell, which is prepared by a method including the following steps in sequence:

[0074] Texturize the surface of the silicon substrate;

[0075] Prepare a selective emitter on the front surface of the silicon substrate according to the preparation method of the selective emitter described in the first aspect of the present application;

[0076] Etch the front and back surfaces of the silicon substrate to remove the silicon oxide layer;

[0077] Perform single-sided etching on the back surface of the silicon substrate to remove the diffusion layer;

[0078] Deposit a passivation layer and / or an antireflection layer on the surface of the silicon substrate; and

[0079] Perform metallization on the surface of the selective emitter and the back surface of the silicon substrate to obtain a front electrode and a back electrode, and the front electrode and the back electrode respectively penetrate the antireflection layer and / or the passivation layer to form an ohmic contact with the silicon substrate.

[0080] Due to the adoption of the selective emitter with excellent contact performance, the solar cell according to the present application exhibits a high conversion efficiency.

[0081] In the above step of texturizing the surface of the silicon substrate, through texturization, the surface mechanical damage during the silicon wafer cutting process can be removed; a textured surface is formed to increase the surface area of the cell, such as forming a positive pyramid type, an inverted pyramid type, a hole-like rough surface made by RIE (reactive ion texturing) or MCT (metal catalyst-assisted texturing), etc.; a light trapping effect is generated to increase the amount of light absorbed by the solar cell and greatly reduce the surface reflectivity of the cell. In an embodiment of the present application, methods such as chemical etching, laser etching, mechanical method, plasma etching, etc. can be used for texturization, and the present application does not make a special limitation on this.

[0082] In addition, before the texturization, the method for preparing the solar cell may further include a step of cleaning the silicon substrate to remove surface metal and organic contaminants.

[0083] In the above steps of preparing the selective emitter, the selective emitter is prepared by the preparation method described in the first aspect of the present application. Among them, first, the surface of the silicon substrate is doped with elements for diffusion to form a diffusion layer and a silicon oxide layer. The sheet resistance and thickness of the diffusion layer under P-type doping conditions can be 150 Ω / sqr to 250 Ω / sqr and 0.4 μm to 0.8 μm respectively, or any specific value within the range, and the thickness of the silicon oxide layer can be 0.07 μm to 0.11 μm or any specific value within the range. Under N-type doping conditions, the sheet resistance and thickness of the diffusion layer can be 150 Ω / sqr to 250 Ω / sqr and 0.2 μm to 0.4 μm respectively, or any specific value within the range, and the thickness of the silicon oxide layer can be 0.04 μm to 0.08 μm or any specific value within the range. Then, laser grooving is performed, and the bottom of the groove reaches the diffusion layer. This laser groove area is the position where the selective emitter is subsequently prepared and is also the position for subsequent metallization to obtain the front electrode (metal grid line electrode). Therefore, several laser grooves can be opened on the front of the silicon substrate in a suitable pattern, shape, and spacing, and the suitable groove width and groove depth can be set, so as to fully collect the photovoltaic current in the best way. Then, the porous silicon in the laser grooving area is removed and at least part of the laser damage is retained. The method can be any one of KOH / H2O2 solution etching, oxidation, hydrofluoric acid cleaning, and plasma irradiation described in the first aspect of the present application. And the diffusion layer in the laser grooving area is subjected to secondary diffusion of the doping elements to form a lightly doped region and a heavily doped region. Under P-type doping conditions, the sheet resistance and thickness of the lightly doped region can be 150 Ω / sqr to 250 Ω / sqr and 0.4 μm to 0.8 μm respectively, or any specific value within the range, and the sheet resistance and thickness of the heavily doped region can be 10 Ω / sqr to 100 Ω / sqr and 0.7 μm to 1.2 μm respectively, or any specific value within the range. Under N-type doping conditions, the sheet resistance and thickness of the lightly doped region can be 150 Ω / sqr to 250 Ω / sqr and 0.2 μm to 0.4 μm respectively, or any specific value within the range, and the sheet resistance and thickness of the heavily doped region can be 10 Ω / sqr to 100 Ω / sqr and 0.4 μm to 1.0 μm respectively, or any specific value within the range. Through the above secondary diffusion, a lightly doped region and a heavily doped region with better doping performance are obtained. Thus, the selective emitter according to the present application is prepared.

[0084] In the steps of etching the silicon oxide layer and etching the diffusion layer on one side, since there is no limitation on whether the doping element diffuses on the back side of the silicon substrate during the preparation of the selective emitter in this application, it is first necessary to etch both sides of the silicon substrate to remove the silicon oxide layers on both sides, and then perform single-sided etching on the back side to remove the diffusion layer on the back side, so that only the diffusion layer on the front side can be retained. In addition, since the silicon oxide layer in the grooving area has been burned off by laser grooving during the preparation of the selective emitter, the etching will not have an etching effect on the selective emitter.

[0085] In the steps of depositing the passivation layer and / or the antireflection layer, methods such as plasma enhanced chemical vapor deposition (PECVD) and metal organic chemical vapor deposition (MOCVD) can be used to deposit the passivation layer and / or the antireflection layer on the surface (front and back) of the silicon substrate. In addition, other methods can also be used, and this application does not make specific limitations on this.

[0086] In a solar cell, the layer that directly contacts the silicon substrate or is relatively close to it is usually called the passivation layer, which mainly functions to passivate the dangling bonds on the substrate surface and prevent the recombination of carriers in the surface region. The layer above the passivation layer and far from the front side of the silicon substrate is called the antireflection layer. Since the antireflection layer is far from the silicon substrate surface, its main purpose is to adjust the refractive index of the overall light-transmitting film layer, reduce light reflection, thereby increasing the amount of light absorbed by the solar cell. In addition, it can further improve the weather resistance of the solar cell and prevent factors such as oxygen, water, and metal ions in the external environment from entering the solar cell to cause defects and lead to a decrease in conversion efficiency. That is to say, the passivation layer and the antireflection layer are functionally divided, and they can have the same or similar substance compositions. Specifically, the passivation layer and the antireflection layer can be composed of a stacked film, and the stacked film can include substances such as aluminum oxide, silicon oxide, silicon oxynitride, silicon nitride, gallium oxide, silicon carbide, amorphous silicon, silicon oxycarbide, or other substances with similar functions. This application does not make specific limitations on this. In addition, hydrogen atoms can be doped into the passivation layer to optimize the passivation effect. And since the subsequent metallization process can burn through the passivation layer and the antireflection layer when forming the front and back electrodes with metal, there is no need to mask the positions of the selective emitter and the back electrode during the deposition of the passivation layer and / or the antireflection layer, and the entire silicon substrate can be deposited.

[0087] In the above-mentioned metallization treatment step, metallization treatment is performed at the laser groove (i.e., at the selective emitter) to form a front electrode (metal grid electrode), and metallization treatment is performed on the side of the silicon substrate that does not include the selective emitter to form a back electrode, and the front electrode and the back electrode are determined to be the front electrode and the back electrode respectively according to the type of the silicon substrate. In addition, the shape and arrangement of the front electrode and the back electrode can be suitably designed and selected according to the needs of the solar cell, and this application does not make any special limitation on this.

[0088] In the embodiment of the present application, the metallization treatment can be achieved by coating a conductive paste, followed by drying and sintering. The coating can be performed by, for example, screen printing technology, etc., but the present application is not limited thereto. The conductive paste can be any one of silver paste, aluminum-containing silver paste or aluminum paste, but the present application is not limited thereto.

[0089] In addition, although a passivation layer and / or an anti-reflection layer are deposited on the selective emitter and the back of the silicon substrate, during the sintering process after the conductive paste is applied, it can penetrate the passivation layer and / or the anti-reflection layer and directly contact the silicon substrate (ohmic contact), thereby achieving electrical connection, and can collect and lead out the carriers generated by the photoelectric effect. Among them, the front electrode is in direct contact with the heavily doped region of the silicon substrate, and the back electrode is in direct contact with the back of the silicon substrate.

[0090] In the present application, since the porous silicon in the laser groove is removed and at least part of the laser damage is retained during the preparation of the selective emitter, the contact area with the metal electrode is increased, the contact resistance is greatly reduced and the recombination current is reduced, thereby improving the conversion efficiency of the solar cell.

[0091] In a third aspect, the present application provides a solar cell, which can be obtained by the preparation method adopted in the second aspect, and can include a front electrode (metal grid electrode), a front anti-reflection layer and / or a passivation layer, a selective emitter prepared according to the preparation method described in the first aspect, a silicon substrate, a back passivation layer and a back electrode arranged in sequence from top to bottom. The selective emitter removes the porous silicon in the laser groove and retains at least part of the laser damage, thereby having excellent ohmic contact with the front electrode (metal grid electrode), and greatly reducing the recombination current, thereby being able to improve the conversion efficiency of the solar cell.

[0092] In addition, without violating the technical purpose and aim of the present application, the solar cell according to the present application may also include special components and structures at any position as required, and the present application does not impose any special limitation on this.

[0093] Fourthly, the present application provides a photovoltaic module, which may include, for example, glass, encapsulant material, at least one solar cell according to the second or third aspect of the present application, encapsulant material, and a backsheet, arranged in sequence from top to bottom. Among them, the solar cells according to the second or third aspect of the present application are usually multiple and are electrically connected in series or parallel. The encapsulant material may be commonly used encapsulant materials in the art such as EVA and POE.

[0094] In addition, with reference to Figure 1 , according to an exemplary embodiment of the present application, a method for preparing a selective emitter is provided, which sequentially includes the following steps:

[0095] S1: Diffuse doping elements on the surface of the silicon substrate 1 to form a diffusion layer 2 and a silicon oxide layer 3 from the inside to the outside on the surface of the silicon substrate 1, wherein the diffusion layer 2 includes doping elements with a first doping concentration;

[0096] S2: Use a laser to groove partial areas on the surface of the silicon oxide layer 3 on one side of the silicon substrate 1, and make the depth of the groove bottom reach the diffusion layer 2;

[0097] S3: Remove the porous silicon 41 in the laser grooving area 4 and retain at least part of the laser damage 42; and

[0098] S4: Perform secondary diffusion of the doping elements on the diffusion layer 2 in the laser grooving area 4, so that the diffusion layer 2 includes a lightly doped region 5 with a first doping concentration and a heavily doped region 6 with a second doping concentration, wherein the heavily doped region 6 corresponds to the laser grooving area 4, and the second doping concentration is greater than the first doping concentration.

[0099] The selective emitter prepared according to the preparation method of the present application can improve the contact performance between the metal grid electrode and the silicon substrate, reduce the contact resistance, and thus improve the conversion efficiency of the corresponding solar cell.

[0100] The technical solutions of the present application are described by way of specific examples as follows:

[0101] Each compound used in the present application is commercially available or commercially ordered, and those skilled in the art can commercially obtain it according to needs.

[0102] Example 1

[0103] The following method for preparing a selective emitter according to the present application is used to prepare a selective emitter;

[0104] Dope and diffuse boron elements on the surface of an N-type silicon substrate to form a diffusion layer with a diffusion thickness of 0.6 μm and a sheet resistance of 170 Ω / sqr and a silicon oxide layer with a thickness of 0.09 μm from the inside to the outside on the surface;

[0105] Use a laser to groove a partial area on the surface of the silicon oxide layer on one side of the silicon substrate, and make the bottom of the groove reach the diffusion layer;

[0106] Etch the grooves opened by the laser with a solution containing 2% by mass of KOH and 4% by mass of H2O2 for 2 min under the temperature condition of 80 °C; and

[0107] Perform secondary diffusion of the doping element on the diffusion layer within the laser grooving area to form a heavily doped region with a thickness of 1.0 μm and a sheet resistance of 80 Ω / sqr and a second doping concentration greater than the first doping concentration at the diffusion layer within the laser grooving area, and make the part of the diffusion layer between the laser grooving areas become a lightly doped region (with a thickness of 0.8 μm and a sheet resistance of 200 Ω / sqr), thereby obtaining a selective emitter according to the present application.

[0108] Refer to Figure 2 and 3 , which are scanning electron microscope (SEM) images of the bottoms of two grooves after the above laser grooving is completed and before the porous silicon is removed. In these two surface topography maps of the groove bottoms, a large number of fluffy porous structures can be clearly seen, which is the porous silicon. The surface containing this porous silicon is not easy to passivate, and the metal recombination current deposited on this surface is relatively large, which will not be conducive to the improvement of the conversion efficiency of the solar cell.

[0109] Refer to Figure 4 and 5 , which are scanning electron microscope (SEM) images of the bottoms of two grooves after the above step of removing the porous silicon is completed. In these two surface topography maps of the groove bottoms, after removing the porous silicon structure, a part of the pit-shaped laser damage structure still remains on the surface of the groove bottom. These damage structures are relatively easy to passivate, and can also improve the contact performance between the metal electrode and the semiconductor, thereby improving the conversion efficiency of the solar cell.

[0110] The effects of the other two methods for removing porous silicon in the present application are similar to those shown in the above Figure 4 and 5 , and will not be repeated here.

[0111] Example 2

[0112] A solar cell, which comprises a metal grid electrode, a front antireflection layer and / or passivation layer, a selective emitter prepared by the preparation method described in the above-mentioned Embodiment 1, a silicon substrate, a back passivation layer and a back electrode arranged in sequence, and is prepared by a method sequentially comprising the following steps:

[0113] Clean and texture the surface of the silicon substrate;

[0114] Prepare a selective emitter on the front surface of the silicon substrate by using the preparation method described in Embodiment 1;

[0115] Etch the front and back surfaces of the silicon substrate to remove the silicon oxide layer;

[0116] Perform single-sided etching on the back surface of the silicon substrate to remove the diffusion layer;

[0117] Deposit a silicon nitride layer on the surface of the silicon substrate by PECVD method as a passivation layer and an antireflection layer; and

[0118] Use screen printing to coat silver paste at the laser grooving positions and on the back surface of the silicon substrate, dry and sinter to form a front electrode and a back electrode, thereby obtaining a solar cell.

[0119] Comparative Example 1

[0120] A solar cell is obtained in the same manner as in Embodiment 2, except that the step of removing porous silicon is not performed when using the preparation method of the selective emitter in Embodiment 1. In the solar cell thus obtained, a considerable amount of porous silicon exists between the metal electrode and the selective emitter.

[0121] Comparative Example 2

[0122] A solar cell is obtained in the same manner as in Embodiment 2, except that the step of removing porous silicon when using the preparation method of the selective emitter in Embodiment 1 is to etch the grooves opened by laser with a solution containing 20% by mass of KOH and 40% by mass of H2O2 at a temperature of 80 °C for 2 min. In the solar cell thus obtained, not only the porous silicon in the laser grooves is removed, but also the laser damage under the porous silicon is removed.

[0123] Measure the performance of the solar cells of Embodiment 2 and Comparative Examples 1 and 2, and the results are shown in Table 1 below.

[0124] [Table 1]

[0125]

[0126] As can be seen from Table 1 above, since the porous silicon is removed and part of the laser damage is retained when fabricating the selective emitter of the solar cell according to Embodiment 2 of the present application, the fill factor and conversion efficiency are both improved. In contrast, in Comparative Example 1, the operation of removing porous silicon is not performed at all, and in Comparative Example 2, deeper removal of both the porous silicon and the laser damage thereunder is carried out. The fill factor and conversion efficiency of these two are both inferior to those of Embodiment 2 of the present application.

[0127] The above specific embodiments of the present application are only preferred embodiments for explaining the present application, rather than limiting the present application. After reading this specification, those skilled in the art can make modifications that do not contribute creatively according to needs. However, any modifications, equivalent replacements, improvements, etc., made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A method for preparing a selective emitter, characterized in that, The following steps are included in sequence: Diffusion of doping elements on the surface of the silicon substrate to form a diffusion layer and a silicon oxide layer from inside to outside on the surface of the silicon substrate, wherein the diffusion layer includes doping elements with a first doping concentration; Using laser to make grooves in a partial area of ​​the surface of the silicon oxide layer on one side of the silicon substrate, and making the bottom of the groove reach the diffusion layer, and stopping laser burning after the diffusion layer is exposed at the bottom of the groove; removing porous silicon in the laser grooved region and preserving at least a portion of the laser damage; and Performing secondary diffusion of the doping element on the diffusion layer in the laser grooved region, so that the diffusion layer includes a lightly doped region with a first doping concentration and a heavily doped region with a second doping concentration, wherein the heavily doped region corresponds to the laser grooved region, and the second doping concentration is greater than the first doping concentration; The method of removing the porous silicon in the laser grooved area and retaining at least part of the laser damage comprises the following steps: etching the laser grooved area for 1 min to 3 min at a temperature of 60° C. to 80° C. using a solution containing 1% to 5% KOH and 3% to 5% H2O2; Alternatively, when the porous silicon in the laser grooved area is removed and at least part of the laser damage is retained, the following steps are included: oxidizing the laser grooved area at a temperature of 800° C. to 900° C. and in an oxygen atmosphere for 30 min to 60 min, and then cleaning with hydrofluoric acid having a mass concentration of 2% to 8% for 3 min to 6 min; Alternatively, the method of removing porous silicon in the laser grooved area and retaining at least part of the laser damage comprises the following steps: irradiating the laser grooved area with a plasma output power of 1000 W to 1500 W for 5 min to 10 min.

2. The preparation method according to claim 1, characterized in that, When the silicon substrate is an N-type silicon substrate, the diffusion layer is a P+ doped layer, and the doping element is at least one of boron, aluminum, gallium or indium.

3. The preparation method according to claim 2, characterized in that, The sheet resistance of the diffusion layer is 150 Ω / sqr~250 Ω / sqr; the diffusion thickness of the diffusion layer is 0.4 μm~0.8 μm; and the thickness of the silicon oxide layer is 0.07 μm~0.11 μm; The square resistance of the lightly doped region is 150 Ω / sqr~250 Ω / sqr, the thickness of the lightly doped region is 0.4 μm~0.8 μm, the square resistance of the heavily doped region is 10 Ω / sqr~100 Ω / sqr; and the thickness of the heavily doped region is 0.7 μm~1.2 μm.

4. The preparation method according to claim 1, characterized in that, When the silicon substrate is a P-type silicon substrate, the diffusion layer is an N+ doped layer, and the doping element is at least one of phosphorus, arsenic or antimony.

5. The preparation method according to claim 4, characterized in that, The sheet resistance of the diffusion layer is 150 Ω / sqr~250 Ω / sqr; the diffusion thickness of the diffusion layer is 0.2 μm~0.4 μm; and the thickness of the silicon oxide layer is 0.04 μm~0.08 μm; The sheet resistance of the lightly doped region is 150 Ω / sqr to 250 Ω / sqr, and the thickness of the lightly doped region is 0.2 μm to 0.4 μm; the sheet resistance of the heavily doped region is 10 Ω / sqr to 100 Ω / sqr; and the thickness of the heavily doped region is 0.4 μm to 1.0 μm.

6. A solar cell, characterized in that, It is obtained by a method sequentially including the following steps: Texturing the surface of the silicon substrate; Preparing a selective emitter on the front surface of the silicon substrate according to the preparation method described in any one of claims 1 to 5; Etching the front and back surfaces of the silicon substrate to remove the silicon oxide layer; Performing single-sided etching on the back surface of the silicon substrate to remove the diffusion layer; Depositing a passivation layer and / or an antireflection layer on the surface of the silicon substrate; And Performing metallization on the surface of the selective emitter and the back surface of the silicon substrate to obtain a front electrode and a back electrode, and the front electrode and the back electrode respectively penetrate the antireflection layer and / or the passivation layer to form an ohmic contact with the silicon substrate.

7. A solar cell, characterized in that, The solar cell is prepared by the method described in claim 6, and includes a front electrode, a front antireflection layer and / or a passivation layer, a selective emitter prepared by the preparation method described in any one of claims 1 to 5, a silicon substrate, a back passivation layer and a back electrode arranged in sequence from top to bottom.

8. A photovoltaic module, characterized in that, The photovoltaic module includes glass, encapsulating material, at least one solar cell according to claim 6 or 7, encapsulating material and a backsheet arranged in sequence from top to bottom.

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