Photovoltaic cell and method of manufacture

Abstract

The application relates to a photovoltaic cell and a preparation method. The application uses trimethylaluminum, oxygen and water vapor to form multiple aluminum oxide passivation layers on a substrate in sequence. The ratio of the oxygen flow rate and the water vapor flow rate for forming a later aluminum oxide passivation layer is smaller than the ratio of the oxygen flow rate and the water vapor flow rate for forming an earlier aluminum oxide passivation layer, so that the refractive index of the later formed aluminum oxide passivation layer is smaller than the refractive index of the earlier formed aluminum oxide passivation layer. The refractive index of the prepared multiple aluminum oxide passivation layers decreases from the side close to the substrate to the side far from the substrate, so that the prepared multiple aluminum oxide passivation layers have a better antireflection effect, and the photoelectric conversion efficiency of the finally prepared photovoltaic cell can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photovoltaic, in particular to a photovoltaic cell and a preparation method thereof. BACKGROUND

[0002] The photoelectric conversion efficiency of the photovoltaic cell in the prior art is low. SUMMARY

[0003] Therefore, the present application provides a photovoltaic cell and a preparation method thereof, which can improve the photoelectric conversion efficiency of the photovoltaic cell.

[0004] In a first aspect, the present application provides a preparation method of a photovoltaic cell, which comprises:

[0005] Step S1: introducing a precursor into a reaction chamber, wherein a substrate has been placed in the reaction chamber, and the precursor comprises trimethylaluminum.

[0006] Step S2: introducing a protective gas into the reaction chamber and pumping out the remaining precursor and by-products.

[0007] Step S3: introducing a post-precursor into the reaction chamber to form an aluminum oxide passivation layer on the substrate, wherein the post-precursor comprises oxygen and water vapor.

[0008] Step S4: introducing a protective gas into the reaction chamber and pumping out the remaining post-precursor and by-products.

[0009] Steps S1, S2, S3 and S4 are repeatedly performed to form at least two aluminum oxide passivation layers on the substrate, wherein the number of cycles is at least two groups of cycles, and the ratio of the oxygen flow rate and the water vapor flow rate in the next group of cycles is less than the ratio of the oxygen flow rate and the water vapor flow rate in the previous group of cycles.

[0010] Step S5: forming an anti-reflection layer on the side of the outermost aluminum oxide passivation layer away from the substrate.

[0011] Step S6: annealing treatment.

[0012] Optionally, the method of making the ratio of the oxygen flow rate and the water vapor flow rate in the next group of cycles less than the ratio of the oxygen flow rate and the water vapor flow rate in the previous group of cycles satisfies any one of the following settings:

[0013] Setting (1): making the oxygen flow rate in the next group of cycles less than the oxygen flow rate in the previous group of cycles, and making the water vapor flow rate in the next group of cycles the same as the water vapor flow rate in the previous group of cycles.

[0014] Setting (2): making the oxygen flow rate in the next group of cycles the same as the oxygen flow rate in the previous group of cycles, and making the water vapor flow rate in the next group of cycles greater than the water vapor flow rate in the previous group of cycles.

[0015] Setting (3): the oxygen flow rate of the next group of cycles is less than the oxygen flow rate of the previous group of cycles, and the water vapor flow rate of the next group of cycles is greater than the water vapor flow rate of the previous group of cycles.

[0016] Optionally, the method of making the ratio of the oxygen flow rate and the water vapor flow rate of the next group of cycles less than the ratio of the oxygen flow rate and the water vapor flow rate of the previous group of cycles satisfies at least one of the following settings:

[0017] Setting (a): the ratio of the oxygen flow rate and the water vapor flow rate within the first group of cycles is 1.8-2.2.

[0018] Setting (b): the ratio of the oxygen flow rate and the water vapor flow rate within the last group of cycles is 0.3-0.7.

[0019] Optionally, the method of making the ratio of the oxygen flow rate and the water vapor flow rate of the next group of cycles less than the ratio of the oxygen flow rate and the water vapor flow rate of the previous group of cycles includes:

[0020] The ratio of the oxygen flow rate and the water vapor flow rate of the next group of cycles has a difference from the ratio of the oxygen flow rate and the water vapor flow rate of the previous group of cycles, and the absolute value of the next difference is less than the absolute value of the previous difference.

[0021] Optionally, the same group of cycles includes at least two cycles, and the ratio of the oxygen flow rate and the water vapor flow rate of the next cycle is the same as the ratio of the oxygen flow rate and the water vapor flow rate of the previous cycle.

[0022] Optionally, the preparation method includes six groups of cycles, and each group of cycles includes five cycles.

[0023] Optionally, any one cycle satisfies at least one of the following settings:

[0024] Setting (a): the time T1 of a single time of introducing the precursor into the reaction chamber is 5S-10S.

[0025] Setting (β): the time T2 of a single time of introducing the post-cursor into the reaction chamber is 5S-10S.

[0026] Setting (γ): the time T3 of a single time of introducing the protective gas into the reaction chamber is 10S-15S.

[0027] Optionally, before performing step S1, the preparation method further includes:

[0028] Step S01: introducing water vapor into the reaction chamber, wherein the reaction chamber has been placed with a substrate;

[0029] Step S02: introducing a protective gas into the reaction chamber and extracting the remaining water vapor.

[0030] Optionally, the step S01 and the step S02 are executed cyclically, and the number of cycles is three to seven cycles.

[0031] In a second aspect, the present application provides a photovoltaic cell, which is prepared by the method for preparing a photovoltaic cell described above, and the photovoltaic cell comprises a silicon substrate and at least two adjacent aluminum oxide passivation layers, and the refractive index of the at least two adjacent aluminum oxide passivation layers decreases along the direction of the silicon substrate to the outermost aluminum oxide passivation layer.

[0032] The concentration of aluminum oxide in the later-formed aluminum oxide passivation layer is lower than that in the previously-formed aluminum oxide passivation layer, and the refractive index of the later-formed aluminum oxide passivation layer is lower than that of the previously-formed aluminum oxide passivation layer, that is, the refractive index of the prepared aluminum oxide passivation layer decreases from the inside to the outside, and accordingly, the prepared aluminum oxide passivation layer has a better antireflection effect, the efficiency of the incidence of sunlight from the aluminum oxide passivation layer into the silicon substrate is higher, the efficiency of the generation of free electrons and holes in the silicon substrate is higher, and the photoelectric conversion efficiency of the prepared photovoltaic cell is higher. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0034] Figure 1 It is a schematic diagram of the structure of the substrate in a specific embodiment.

[0035] Figure 2 It is a schematic diagram of the structure of the substrate with an aluminum oxide passivation layer formed on the surface.

[0036] Figure 3 It is a schematic diagram of the local structure of the aluminum oxide passivation layer.

[0037] Figure 4 It is a schematic diagram of the structure of the photovoltaic cell in a specific embodiment.

[0038] 10-photovoltaic cell;

[0039] 1-N-type silicon substrate;

[0040] 2-P-type diffusion layer;

[0041] 3-tunneling layer;

[0042] 4-N-type doped dielectric layer;

[0043] 5 - Alumina passivation layer, 51 - first alumina passivation layer, 52 - second alumina passivation layer, 53 - third alumina passivation layer, 54 - fourth alumina passivation layer, 55 - fifth alumina passivation layer, 56 - sixth alumina passivation layer;

[0044] 6 - Top antireflection layer;

[0045] 7 - Back antireflection layer;

[0046] 8 - Negative electrode grid line;

[0047] 9 - Positive electrode grid line. DETAILED DESCRIPTION

[0048] In order to better understand the technical solutions of the present application, the embodiments of the present application are described in detail below in conjunction with the accompanying drawings. It should be clear that the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application. The terms used in the embodiments of the present application are only for the purpose of describing the specific embodiments, and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should be understood that the term "and / or" used herein is only to describe the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0049] The direction X and the direction Y in the drawings herein are perpendicular, and the direction from the back surface of the photovoltaic cell to the top surface is the direction X.

[0050] In a first aspect, the embodiments of the present application provide a preparation method of a photovoltaic cell, which comprises the following contents:

[0051] Step S0: placing a substrate into a reaction chamber.

[0052] The substrate can include, for example, Figure 1The N-type silicon substrate 1, the P-type diffusion layer 2, the tunneling layer 3 and the N-type doped dielectric layer 4 are shown. The P-type diffusion layer 2 is located on one side of the N-type silicon substrate 1, the tunneling layer 3 is located on the other side of the N-type silicon substrate 1, and the N-type doped dielectric layer 4 is located on the side of the tunneling layer 3 away from the N-type silicon substrate 1. The N-type silicon substrate 1 can be doped with at least one N-type element (fifth main group element in the periodic table of chemical elements), such as phosphorus, arsenic, antimony, etc. The P-type diffusion layer 2 can be doped with at least one P-type element (third main group element in the periodic table of chemical elements), such as boron, gallium, aluminum, etc. A P-N junction is formed between the N-type silicon substrate 1 and the P-type diffusion layer 2. The tunneling layer 3 can include at least one of silicon oxide, silicon nitride and silicon oxynitride. The N-type doped dielectric layer 4 can also be doped with at least one N-type element, and before annealing, the N-type doped dielectric layer 4 can include at least one of polysilicon, amorphous silicon and microcrystalline silicon. As shown in the figure Figure 1 The preparation process of the substrate shown can include texturing, boron diffusion, backside etching, preparation of a tunneling passivation contact structure, and front side etching.

[0053] In other embodiments (not shown in the figure), the substrate can also have other configurations. Therefore, the preparation method provided by the embodiments of the present application can also be applied to any photovoltaic cell that needs aluminum oxide as a passivation layer.

[0054] The following content is mainly described with reference to the substrate shown in the figure.

[0055] In addition, the reaction chamber is used to provide an environment in which an atomic layer thin film deposition (ALD) process can be performed, so that in the subsequent process, the P-type diffusion layer 2 of the substrate can be formed with an aluminum oxide passivation layer 5 on the surface thereof away from the N-type silicon substrate 1. Figure 2 The aluminum oxide passivation layer 5 is shown. The process of forming the aluminum oxide passivation layer 5 includes the following steps S1, S2, S3 and S4.

[0056] Step S1: introducing a precursor into the reaction chamber, wherein the precursor includes trimethylaluminum (TMA), and the chemical formula of trimethylaluminum is Al(CH3)3(g). In this setting, the hydroxyl group on the surface of the P-type diffusion layer 2 away from the N-type silicon substrate 1 (described as the top surface of the P-type diffusion layer 2 in the subsequent process) can react with trimethylaluminum to generate an intermediate and a byproduct, the chemical formula of the intermediate is (Al-O)2-Al(CH3)*(s), and the byproduct is methane (CH4).

[0057] Step S2: Introduce a protective gas into the reaction chamber while simultaneously extracting the remaining precursors and byproducts. After the remaining precursors and byproducts are extracted, fill the reaction chamber with the protective gas to ensure the substrate is in a protective gas environment. Once the reaction chamber is filled with the protective gas, stop the extraction. The protective gas may include nitrogen and / or an inert gas (e.g., helium, argon, or krypton). All protective gases mentioned later will be described in the following section.

[0058] Step S3: Introduce a precursor, comprising oxygen and water vapor, into the reaction chamber. Under this setup, the oxygen and water vapor react with the intermediate located on the top surface of the P-type diffusion layer 2 to form an alumina passivation layer, while simultaneously generating the byproduct methane (CH4).

[0059] Step S4: Introduce protective gas into the reaction chamber while simultaneously extracting the remaining precursors and byproducts. After the remaining precursors and byproducts are extracted, fill the reaction chamber with protective gas to ensure the substrate is in a protective gas environment. Once the reaction chamber is filled with protective gas, stop the extraction process.

[0060] Steps S1, S2, S3, and S4 are repeated cyclically to form at least two aluminum oxide passivation layers on the top surface of the P-type diffusion layer 2, for example, as... Figure 3 As shown, a first aluminum oxide passivation layer 51 is formed first, then a second aluminum oxide passivation layer 52 is formed, followed by a third aluminum oxide passivation layer 53, then a fourth aluminum oxide passivation layer 54, and then a fifth aluminum oxide passivation layer 55 and a sixth aluminum oxide passivation layer 66 are formed in sequence.

[0061] Specifically, the number of cycles for steps S1, S2, S3, and S4 is at least two sets, such that the ratio of oxygen flow rate to water vapor flow rate in the latter set of cycles is less than that in the former set of cycles. Under this setting, the alumina concentration in the alumina passivation layer formed in the latter set of cycles (e.g., the second alumina passivation layer 52) is lower than the alumina concentration in the alumina passivation layer formed in the former set of cycles (e.g., the first alumina passivation layer 51), resulting in a lower refractive index. This can also be understood as the refractive index of the prepared alumina passivation layer 5 decreasing from the inside to the outside along direction X. This setting allows the prepared alumina passivation layer 5 to have better anti-reflection effects, resulting in higher efficiency of sunlight incident from the alumina passivation layer 5 into the silicon substrate 1, higher efficiency of generating free electrons and holes within the silicon substrate 1, and thus higher photoelectric conversion efficiency of the prepared photovoltaic cell.

[0062] The oxygen has strong oxidation ability and strong molecular structure stability, and the oxidation of the oxygen has a great influence on the rate of forming the aluminum oxide. The concentration of the oxygen can be controlled to control the concentration of the aluminum oxide in the aluminum oxide passivation layer. If the concentration of the oxygen is relatively low, the concentration of the aluminum oxide in the formed aluminum oxide passivation layer will also be relatively low, that is, the refractive index of the formed aluminum oxide passivation layer is low. From another point of view, if the concentration of the oxygen is relatively low, the density of the aluminum oxide in the formed aluminum oxide passivation layer will also be relatively low, that is, the formed aluminum oxide passivation layer is relatively loose, and the refractive index of the relatively loose aluminum oxide passivation layer is low.

[0063] In addition, the preparation method provided by the embodiment of the application can include two cycles, three cycles, four cycles, five cycles, six cycles or seven cycles.

[0064] Optionally, the method of making the ratio of the oxygen flow rate and the water vapor flow rate of the next cycle smaller than the ratio of the oxygen flow rate and the water vapor flow rate of the previous cycle meets at least one of the following settings:

[0065] Setting (1): making the oxygen flow rate of the next cycle smaller than the oxygen flow rate of the previous cycle, and making the water vapor flow rate of the next cycle the same as the water vapor flow rate of the previous cycle.

[0066] Setting (2): making the oxygen flow rate of the next cycle the same as the oxygen flow rate of the previous cycle, and making the water vapor flow rate of the next cycle greater than the water vapor flow rate of the previous cycle.

[0067] Setting (3): making the oxygen flow rate of the next cycle smaller than the oxygen flow rate of the previous cycle, and making the water vapor flow rate of the next cycle greater than the water vapor flow rate of the previous cycle.

[0068] Settings (1), (2) and (3) can all achieve the refractive index of the aluminum oxide passivation layer 5 to be prepared gradually decreasing from inside to outside along the direction X, so that the prepared aluminum oxide passivation layer 5 has a better antireflection effect.

[0069] Optionally, the method of making the ratio of the oxygen flow rate and the water vapor flow rate of the next cycle smaller than the ratio of the oxygen flow rate and the water vapor flow rate of the previous cycle meets at least one of the following settings:

[0070] Setting (a): the ratio of oxygen flow rate and water vapor flow rate in the first group of cycles is 1.8-2.2. Specifically, the ratio can be 1.8, 1.9, 2.0, 2.1 or 2.2. Since the aluminum oxide passivation layer formed by the first group of cycles (e.g. the first aluminum oxide passivation layer 51) is closest to the P-type diffusion layer 2, if the ratio is less than 1.8, the concentration of aluminum oxide in the aluminum oxide passivation layer formed by the first group of cycles is low, and accordingly the density of negative charges in the aluminum oxide passivation layer formed by the first group of cycles is too low, and the density of positive charges induced by negative charges in the part of the P-type diffusion layer 2 close to the aluminum oxide passivation layer formed by the first group of cycles is also too low. Accordingly, the built-in electric field formed by the aluminum oxide passivation layer and the P-type diffusion layer 2 is too weak, and the minority carriers (free electrons) in the P-type diffusion layer 2 are not easy to move away from the contact surface between the P-type diffusion layer 2 and the aluminum oxide passivation layer under the action of the weak built-in electric field, and the minority carriers (free electrons) in the P-type diffusion layer 2 are easy to recombine with holes at the contact surface, thereby making the photoelectric conversion efficiency of the prepared photovoltaic cell too low. If the ratio is greater than 2.2, the concentration of aluminum oxide in the aluminum oxide passivation layer formed by the first group of cycles is too high, and accordingly the refractive index of the aluminum oxide passivation layer formed by the first group of cycles is too large, and in turn the reflection of light by the aluminum oxide passivation layer formed by the first group of cycles is too strong, and the efficiency of light incident from the aluminum oxide passivation layer formed by the first group of cycles into the silicon substrate is too low, and the efficiency of generating free electrons and holes in the silicon substrate is too low, making the photoelectric conversion efficiency of the prepared photovoltaic cell too low. Therefore, the ratio of oxygen flow rate and water vapor flow rate in the first group of cycles is preferably in the range of 1.8-2.2.

[0071] Setting (b): the ratio of oxygen flow rate and water vapor flow rate in the last group of cycles is 0.3-0.7. Specifically, the ratio can be 0.3, 0.4, 0.5, 0.6 or 0.7. Since the aluminum oxide passivation layer formed by the last group of cycles (e.g. the sixth aluminum oxide passivation layer 56) is farthest from the P-type diffusion layer 2, if the ratio is greater than 0.7, the concentration of aluminum oxide in the aluminum oxide passivation layer formed by the last group of cycles is too high, and accordingly the refractive index of the aluminum oxide passivation layer formed by the last group of cycles is too large, and in turn the reflection of light by the aluminum oxide passivation layer formed by the last group of cycles is too strong, and the efficiency of light incident from the aluminum oxide passivation layer formed by the last group of cycles into the inner aluminum oxide passivation layer is too low, and the efficiency of light incident into the silicon substrate is too low, and the efficiency of generating free electrons and holes in the silicon substrate is too low, making the photoelectric conversion efficiency of the prepared photovoltaic cell too low. If the ratio is less than 0.3, the rate of forming the aluminum oxide passivation layer in the last group of cycles is slow, resulting in a long preparation time. Therefore, the ratio of oxygen flow rate and water vapor flow rate in the last group of cycles is preferably in the range of 0.3-0.7.

[0072] Optionally, the method of making the ratio of oxygen flow rate and water vapor flow rate of the latter group of cycles smaller than the ratio of oxygen flow rate and water vapor flow rate of the former group of cycles comprises: making the ratio of oxygen flow rate and water vapor flow rate of the latter group of cycles and the ratio of oxygen flow rate and water vapor flow rate of the former group of cycles have a difference, and making the absolute value of the latter difference smaller than the absolute value of the former difference. For example, making the ratio of oxygen flow rate and water vapor flow rate of the second group of cycles and the ratio of oxygen flow rate and water vapor flow rate of the first group of cycles have a first difference, making the ratio of oxygen flow rate and water vapor flow rate of the third group of cycles and the ratio of oxygen flow rate and water vapor flow rate of the second group of cycles have a second difference, and making the absolute value of the second difference smaller than the absolute value of the first difference. In this way, the absolute value of the third difference is smaller than the absolute value of the second difference, and the absolute value of the last difference is smaller than the absolute value of the second last difference. Accordingly, the difference between the refractive index of the last formed aluminum oxide passivation layer (for example, the sixth aluminum oxide passivation layer 56) and the refractive index of the second last formed aluminum oxide passivation layer (the fifth aluminum oxide passivation layer 55) is relatively small, and the difference between the refractive index of the second last formed aluminum oxide passivation layer (the fifth aluminum oxide passivation layer 55) and the refractive index of the third last formed aluminum oxide passivation layer (the fourth aluminum oxide passivation layer 54) is also relatively small. When the sunlight passes through these aluminum oxide passivation layers with relatively low refractive index and relatively small refractive index change in turn, the reflected part of the sunlight is relatively small, which is conducive to more sunlight entering the relatively deep part of the aluminum oxide passivation layer (for example, the third aluminum oxide passivation layer 53, the second aluminum oxide passivation layer 52, and the first aluminum oxide passivation layer 51), even if the refractive index of the relatively deep part is relatively large, causing a part of the light to be reflected, but the reflected light will be reflected back by the relatively shallow aluminum oxide passivation layer, thus being conducive to the silicon substrate absorbing more light, so that the silicon substrate can generate more free electrons and photo-generated holes, thereby making the prepared photovoltaic cell have a higher photoelectric conversion efficiency.

[0073] In other embodiments, the latter difference can also be the same as the former difference.

[0074] It should be noted that the adjacent two or more cycles with the same ratio of oxygen flow rate and water vapor flow rate can be grouped into the same group of cycles. Therefore, the same group of cycles can include at least two cycles, and in the same group of cycles, the ratio of oxygen flow rate and water vapor flow rate of the latter cycle is the same as that of the former cycle. Therefore, the refractive index of the aluminum oxide passivation layer formed by the latter cycle is the same as that of the aluminum oxide passivation layer formed by the former cycle. In the same group of cycles, since each cycle uses the protective gas to purge the reaction chamber, the by-products in the reaction chamber can be purged in time, so that the aluminum oxide passivation layer with a certain thickness size, high purity and the same refractive index can be formed in the same group of cycles.

[0075] In the same group of cycles, there can be two cycles, three cycles, four cycles, five cycles or six cycles.

[0076] In other embodiments, there can be only one cycle in the group of cycles.

[0077] Optionally, any one cycle satisfies at least one of the following settings:

[0078] Setting (a): the time T1 for a single time of introducing the precursor into the reaction chamber is 5S-10S, and the time T1 can be 5S, 6S, 7S, 8S, 9S or 10S. Setting (a) can ensure that the substrate surface has sufficient time to adsorb trimethylaluminum under the current cycle.

[0079] Setting (b): the time T2 for a single time of introducing the post-cursor into the reaction chamber is 5S-10S, and the time T2 can be 5S, 6S, 7S, 8S, 9S or 10S. Setting (b) can ensure that there is sufficient time to form an aluminum oxide passivation layer under the current cycle.

[0080] Setting (g): the time T3 for a single time of introducing the protective gas into the reaction chamber is 10S-15S, and the time T3 can be 10S, 11S, 12S, 13S, 14S or 15S. Setting (g) can ensure that the remaining precursors, post-cursors and by-products are completely removed under the current cycle.

[0081] Optionally, before step S1 is performed, the preparation method of the embodiments of the present application further includes pretreatment, and the method of the pretreatment includes:

[0082] Step S01: introducing water vapor into the reaction chamber, and the substrate has been arranged in the reaction chamber. Under this setting, the top surface of the P-type diffusion layer 2 of the substrate is wetted, or in other words, the top surface of the P-type diffusion layer 2 of the substrate is provided with liquid water, so that the trimethylaluminum introduced in the subsequent first cycle can be dissolved in the liquid water on the top surface of the P-type diffusion layer 2, which is conducive to the combination of more trimethylaluminum and the hydroxyl group on the top surface of the P-type diffusion layer 2, thereby increasing the rate of generating intermediates.

[0083] In the step S01, the time T3 for a single execution is 4S-6S, and the time T3 can be 4S, 5S or 6S.

[0084] Step S02: introducing the protective gas into the reaction chamber and pumping out the remaining water vapor, and after the remaining water vapor is pumped out, the reaction chamber is filled with the protective gas, so that the substrate is in an environment filled with the protective gas, and after the reaction chamber is filled with the protective gas, the pumping is stopped.

[0085] The execution time T4 for a single step S02 is 4S to 6S, and the specific time T4 can be 10S, 11S or 12S.

[0086] Optionally, steps S01 and S02 are executed cyclically, with the number of cycles being three, four, five, six, or seven. Under this configuration, a larger amount of liquid water can be stored on the top surface of the P-type diffusion layer 2, but without causing excessive condensation of liquid water on the inner wall of the reaction chamber. This allows more water vapor that cannot condense on the top surface of the P-type diffusion layer 2 to be extracted in a timely manner, thereby improving the water vapor recycling rate.

[0087] In other embodiments, steps S01 and S02 may be performed only once during the preprocessing process.

[0088] In other embodiments, the preprocessing steps S01 and S02 may be omitted.

[0089] During the pretreatment process, the preparation method of this application embodiment may include five pretreatment cycles. During the preparation of the alumina passivation layer, the preparation method of this application embodiment may include six sets of cycles, each set of cycles comprising five cycles, which are used to form six alumina passivation layers with different refractive indices. Therefore, the preparation method provided by this application embodiment may include a total of thirty-five cycles, as shown in Table 1 below.

[0090] Table 1

[0091]

[0092] After preparing the required alumina passivation layer 5, the preparation method of this application embodiment further includes:

[0093] Step S5: An antireflective layer is formed on the side of the outermost alumina passivation layer (e.g., the sixth alumina passivation layer 56) facing away from the substrate. For example, as... Figure 4 The top antireflection layer 6 shown comprises silicon nitride and hydrogen. Correspondingly, an antireflection layer is also formed on the side of the substrate opposite to the innermost alumina passivation layer (e.g., the first alumina passivation layer 51). For example, as... Figure 4 The back antireflection layer 7 shown comprises silicon nitride and hydrogen. Both the top antireflection layer 6 and the back antireflection layer 7 reduce the reflection of sunlight, allowing more light to enter the silicon substrate 1 and generating more free electrons and holes within the silicon substrate 1, thereby improving the photoelectric conversion efficiency.

[0094] The process for preparing the anti-reflective layer can be Low Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), or Atmospheric Pressure Chemical Vapor Deposition (APCVD).

[0095] In other embodiments, the material of the anti-reflective layer can also include at least one of titanium oxide, silicon oxide, and cesium oxide.

[0096] Step S6: annealing. For example, the substrate with the multi-layered aluminum oxide passivation layer and the anti-reflective layer is put into an annealing furnace, and the internal stress of the substrate, the aluminum oxide passivation layer, and the anti-reflective layer is eliminated by high temperature, and the hydrogen element in the anti-reflective layer is diffused to the surface of the silicon substrate by high temperature, so that the hydrogen atoms are combined with the dangling bonds on the surface of the silicon substrate, to achieve the effect of passivating the surface of the silicon substrate.

[0097] Step S7: as shown in FIG. 2, the positive gate line 9 is formed to be in metal contact with the P-type diffusion layer 2, and the negative gate line 8 is formed to be in metal contact with the N-type doped dielectric layer 4. The process for forming the gate line can include screen printing-sintering, electroplating, evaporation, laser transfer, etc. Figure 4

[0098] The step S6 can be performed before the step S7, or the step S6 can be performed after the step S7, or the step S6 is performed before the step S7, and the step S6 can be further performed after the step S7.

[0099] In a second aspect, the embodiments of the present application provide a photovoltaic cell, which is prepared by the preparation method described above, and has good anti-reflective advantage and high photoelectric conversion efficiency. The photovoltaic cell includes a silicon substrate and at least two adjacent aluminum oxide passivation layers, and the refractive index of the at least two adjacent aluminum oxide passivation layers decreases along the direction of the silicon substrate pointing to the outermost aluminum oxide passivation layer.

[0100] For example, as shown in the figure, the photovoltaic cell 10 can include an N-type silicon substrate 1, a P-type diffusion layer 2, a tunneling layer 3, an N-type doped dielectric layer 4, an aluminum oxide passivation layer 5, a top anti-reflective layer 6, a back anti-reflective layer 7, a negative gate line 8, and a positive gate line 9.

[0101] ​The back anti-reflection layer 7, the N-type doped dielectric layer 4, the tunneling layer 3, the N-type silicon substrate 1, the P-type diffusion layer 2, the aluminum oxide passivation layer 5, and the top anti-reflection layer 6 are stacked along the direction X. The negative electrode grid line 8 is disposed through the top anti-reflection layer 6 and the aluminum oxide passivation layer 5, the positive electrode grid line 9 is in metal contact with the P-type diffusion layer 2, the negative electrode grid line 8 is disposed through the back anti-reflection layer 7, and the negative electrode grid line 8 is in metal contact with the N-type doped dielectric layer 4.

[0102] The N-type silicon substrate 1 can be doped with at least one N-type element (fifth main group element in the periodic table of chemical elements), such as phosphorus, arsenic, antimony, and the like. The P-type diffusion layer 2 can be doped with at least one P-type element (third main group element in the periodic table of chemical elements), such as boron, gallium, aluminum, and the like. The tunneling layer 3 can include at least one of silicon oxide, silicon nitride, and silicon oxynitride. The N-type doped dielectric layer 4 can also be doped with at least one N-type element, and after annealing, the N-type doped dielectric layer 4 includes polysilicon.

[0103] The N-type silicon substrate 1 and the P-type diffusion layer 2 form a P-N junction. Under the action of light, more free electrons tend to move to the N-type silicon substrate 1 and then to the N-type doped dielectric layer 4, and more holes tend to move to the P-type diffusion layer 2, so that the negative electrode grid line 8 exhibits negative electrical properties, and the positive electrode grid line 9 can exhibit positive electrical properties.

[0104] The contact surface between the P-type diffusion layer 2 and the aluminum oxide passivation layer 5 is passivated by the negative charge field of the aluminum oxide passivation layer 5 and chemically passivated by hydrogen in the top anti-reflection layer 6. The contact surface between the N-type silicon substrate 1 and the tunneling layer 3 is passivated by the tunneling passivation contact structure formed by the tunneling layer 3 and the N-type doped dielectric layer 4. The contact surface between the N-type doped dielectric layer 4 and the back anti-reflection layer 7 is chemically passivated by hydrogen in the back anti-reflection layer 7. In this arrangement, the photovoltaic cell has a high photoelectric conversion efficiency.

[0105] As shown in the drawings, the aluminum oxide passivation layer 5 includes a first aluminum oxide passivation layer 51, a second aluminum oxide passivation layer 52, a third aluminum oxide passivation layer 53, a fourth aluminum oxide passivation layer 54, a fifth aluminum oxide passivation layer 55, and a sixth aluminum oxide passivation layer 56. The first aluminum oxide passivation layer 51 is closest to the P-type diffusion layer 2, and the sixth aluminum oxide passivation layer 56 is closest to the top anti-reflection layer 6. In the aluminum oxide passivation layer 5, the first aluminum oxide passivation layer 51 has the largest refractive index, the sixth aluminum oxide passivation layer 56 has the smallest refractive index, and the refractive index of the aluminum oxide passivation layer 5 decreases along the direction X. In this arrangement, the aluminum oxide passivation layer 5 can effectively reduce the reflection of sunlight, i.e., the sunlight can be more efficiently incident into the silicon substrate 1 through the aluminum oxide passivation layer 5, and the generation of free electrons and holes in the silicon substrate 1 is more efficient, thereby increasing the photoelectric conversion efficiency of the photovoltaic cell.

[0106] The refractive index of the first aluminum oxide passivation layer 51 is in the range of 1.8-2.0, and specifically can be 1.8, 1.9 or 2.0. The refractive index of the second aluminum oxide passivation layer 52 is in the range of 1.6-1.8, and specifically can be 1.6, 1.7 or 1.8. The refractive index of the third aluminum oxide passivation layer 53 is in the range of 1.4-1.6, and specifically can be 1.4, 1.5 or 1.6. The refractive index of the fourth aluminum oxide passivation layer 54 is in the range of 1.2-1.4, and specifically can be 1.2, 1.3 or 1.4. The refractive index of the fifth aluminum oxide passivation layer 55 is in the range of 1.0-1.2, and specifically can be 1.0, 1.1 or 1.2. The refractive index of the sixth aluminum oxide passivation layer 56 is in the range of 0.8-1.0, and specifically can be 0.8, 0.9 or 1.0.

[0107] In other embodiments (not shown in the figures), the photovoltaic cell can also be of other configurations, i.e. the photovoltaic cell satisfies the condition of "having at least two adjacent aluminum oxide passivation layers, and the refractive index of the at least two adjacent aluminum oxide passivation layers decreases along the silicon substrate in the direction pointing to the outermost aluminum oxide passivation layer".

[0108] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a photovoltaic cell, characterized in that, include: Step S1: Introduce a precursor into the reaction chamber, wherein a substrate has been placed in the reaction chamber, and the precursor includes trimethylaluminum; Step S2: Introduce a protective gas into the reaction chamber and extract the remaining precursor and byproducts; Step S3: Introduce a precursor into the reaction chamber to form an alumina passivation layer on the substrate, wherein the precursor includes oxygen and water vapor; Step S4: Introduce protective gas into the reaction chamber and extract the remaining precursor and byproducts; The steps S1, S2, S3 and S4 are executed cyclically to form at least two aluminum oxide passivation layers on the substrate, wherein the number of cycles is at least two sets of cycles, such that the ratio of the oxygen flow rate and the water vapor flow rate in the later set of cycles is less than the ratio of the oxygen flow rate and the water vapor flow rate in the earlier set of cycles. Step S5: Form an antireflection layer on the side of the outermost alumina passivation layer opposite to the substrate; Step S6: Annealing.

2. The method for preparing photovoltaic cells according to claim 1, characterized in that, The method of making the ratio of oxygen flow rate to water vapor flow rate in the subsequent cycle less than the ratio of oxygen flow rate to water vapor flow rate in the previous cycle satisfies any of the following settings: Setting (1): The oxygen flow rate of the subsequent cycle is less than the oxygen flow rate of the previous cycle, and the water vapor flow rate of the subsequent cycle is the same as the water vapor flow rate of the previous cycle; Setting (2): The oxygen flow rate of the subsequent cycle is the same as the oxygen flow rate of the previous cycle, and the water vapor flow rate of the subsequent cycle is greater than the water vapor flow rate of the previous cycle. Setting (3): The oxygen flow rate of the subsequent cycle is less than the oxygen flow rate of the previous cycle, and the water vapor flow rate of the subsequent cycle is greater than the water vapor flow rate of the previous cycle.

3. The method for preparing a photovoltaic cell according to claim 1, characterized in that, The method for making the ratio of oxygen flow rate to water vapor flow rate in the subsequent cycle less than the ratio of oxygen flow rate to water vapor flow rate in the previous cycle satisfies at least one of the following settings: Setting (a): The ratio of the oxygen flow rate to the water vapor flow rate in the first cycle is 1.8 to 2.2; Setting (b): The ratio of the oxygen flow rate to the water vapor flow rate in the last cycle is 0.3 to 0.

7.

4. The method for preparing a photovoltaic cell according to claim 1, characterized in that, The method for making the ratio of the oxygen flow rate to the water vapor flow rate in a subsequent cycle less than the ratio of the oxygen flow rate to the water vapor flow rate in a previous cycle includes: The ratio of oxygen flow rate to water vapor flow rate in the subsequent cycle is such that there is a difference between the ratio of oxygen flow rate to water vapor flow rate in the previous cycle, and the absolute value of the latter difference is less than the absolute value of the former difference.

5. The method for preparing a photovoltaic cell according to any one of claims 1 to 4, characterized in that, The same set of cycles includes at least two cycles, and the ratio of the oxygen flow rate to the water vapor flow rate in the latter cycle is the same as the ratio of the oxygen flow rate to the water vapor flow rate in the former cycle.

6. The method for preparing a photovoltaic cell according to claim 5, characterized in that, The preparation method includes six sets of cycles, each set of cycles comprising five cycles.

7. The method for preparing a photovoltaic cell according to claim 5, characterized in that, Any loop satisfies at least one of the following settings: Setting (α): The time T1 for introducing the precursor into the reaction chamber once is 5S to 10S; Setting (β): The time T2 for a single introduction of the precursor into the reaction chamber is 5S to 10S; Setting (γ): The time T3 for introducing the protective gas into the reaction chamber once is 10S to 15S.

8. The method for preparing a photovoltaic cell according to any one of claims 1 to 4, characterized in that, Before performing step S1, the preparation method further includes: Step S01: Water vapor is introduced into the reaction chamber, wherein a substrate has been placed in the reaction chamber; Step S02: Introduce protective gas into the reaction chamber and extract the remaining water vapor.

9. The method for preparing a photovoltaic cell according to claim 8, characterized in that, Steps S01 and S02 are executed repeatedly for a period of three to seven cycles.

10. A photovoltaic cell, characterized in that, The photovoltaic cell is prepared by the method for preparing a photovoltaic cell according to any one of claims 1 to 9, and the photovoltaic cell includes a silicon substrate and at least two adjacent aluminum oxide passivation layers; The refractive index of at least two adjacent alumina passivation layers decreases along the direction from the silicon substrate toward the outermost alumina passivation layer.

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