Patent Publication Number: US-2023137353-A1

Title: Photovoltaic cell, method for manufacturing same, and photovoltaic module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation in part of U.S. patent application Ser. No. 17/857,169, filed on Jul. 4, 2022, which is a continuation of U.S. patent application Ser. No. 17/386,442, filed on Jul. 27, 2021, which claims the benefit of priority to Chinese Patent Application No. 202011591700.4 filed on Dec. 29, 2020, each of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Generally, the present disclosure relates to a photovoltaic field, in particular to a photovoltaic cell, a method for manufacturing the photovoltaic cell and a photovoltaic module. 
     BACKGROUND 
     A typical passivated emitter and rear cell (PERC) uses stacked aluminum oxide/silicon nitride as a rear passivation layer. An aluminum oxide layer has a relatively high fixed negative charge density, and a large number of the fixed negative charges may shield electrons on a silicon substrate surface, thus reducing the electrons usable for recombination and achieving suppression of carrier recombination on the surface. The PERC has become a mainstream technology in the photovoltaic cell. However, for a PERC based photovoltaic module, a potential induced degradation (PID) effect may negatively affect the performance of the cell, thus resulting in lowered conversion efficiency. An important cause of the PID effect may be that a potential difference between the cells and other structures (such as a packaging material) of the photovoltaic module disturbs a normal current path in the cells during the power generation, then the photovoltaic cell presents undesirable situations such as power attenuation and lower power generation. Therefore, it is desirable to improve an anti-PID effect of the PERC and maintain high efficiency of the PERC. 
     SUMMARY 
     Some embodiments of the present disclosure provide a photovoltaic cell, a method for manufacturing the photovoltaic cell, and a photovoltaic module, which can improve anti-PID performance and power generation efficiency of the photovoltaic cell. 
     In order to solve the above problems, embodiments of the present disclosure provide a photovoltaic cell, including: a substrate; a first passivation layer and a first anti-reflection layer that are sequentially disposed on a front surface of the substrate in a direction away from the substrate; and a second passivation layer, a polarization phenomenon weakening (PPW) layer and at least one silicon nitride layer Si u N v  that are sequentially disposed on a rear surface of the substrate in a direction away from the substrate, where 1&lt;u/v&lt;4. In some embodiments, the second passivation layer includes at least one aluminum oxide layer Al x O y , where 0.8&lt;y/x&lt;1.6, a refractive index of the at least one aluminum oxide layer is in a range of 1.4 to 1.6, and a thickness of the at least one aluminum oxide layer is in a range of 4 nm to 20 nm. In some embodiments, the PPW layer includes at least one silicon oxynitride layer Si r O s N t , where r&gt;s&gt;t, a refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8, and a thickness of the at least one silicon oxynitride layer is in a range of 1 nm to 30 nm. In some embodiments, a refractive index of the at least one silicon nitride layer is in a range of 1.9 to 2.5, and a thickness of the at least one silicon nitride layer is in a range of 50 nm to 100 nm. 
     In some embodiments, the at least one silicon nitride layer includes a first silicon nitride layer, a second silicon nitride layer and a third silicon nitride layer stacked in the direction away from the substrate, wherein a thickness of the first silicon nitride layer is in a range of 5 nm to 20 nm, a thickness of the second silicon nitride layer is in a range of 20 nm to 40 nm, and a thickness of the third silicon nitride layer is in a range of 40 nm to 75 nm. 
     In some embodiments, refractive indexes of the first silicon nitride layer, the second silicon nitride layer and the third silicon nitride layer decrease layer by layer in the direction away from the substrate, a refractive index of the first silicon nitride layer is in a range of between 2.1 to 2.5, a refractive index of the second silicon nitride layer is in a range of 2 to 2.3, and a refractive index of the third silicon nitride layer is in a range of 1.9 to 2.1. 
     In some embodiments, a concentration of silicon atoms in the at least one silicon oxynitride layer is between 5×10 21 /cm 3  and 2.5×10 22 /cm 3 . 
     In some embodiments, the second passivation layer further includes a silicon oxide layer, and the silicon oxide layer is disposed between the substrate and the at least one aluminum oxide layer. 
     In some embodiments, a thickness of the silicon oxide layer is in a range of 0.1 nm to 5 nm. 
     Embodiments of the present disclosure further provide a method for manufacturing a photovoltaic cell, including: providing a substrate; forming a first passivation layer and a first anti-reflection layer sequentially on a front surface of the substrate in a direction away from the substrate; and forming a second passivation layer, a polarization phenomenon weakening (PPW) layer and at least one silicon nitride layer Si u N v  sequentially on a rear surface of the substrate in a direction away from the substrate, wherein 1&lt;u/v&lt;4; wherein the second passivation layer includes at least one aluminum oxide layer Al x O y , wherein 0.8&lt;y/x&lt;1.6 and a refractive index of the at least one layer aluminum oxide layer is in a range of 1.4 to 1.6, and a thickness of the at least one aluminum oxide layer is in a range of 4 nm to 20 nm; wherein the PPW layer includes at least one silicon oxynitride layer Si r O s N t , wherein r&gt;s&gt;t and a refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8, and a thickness of the at least one silicon oxynitride layer is in a range of 1 nm to 30 nm; wherein a refractive index of the at least one silicon nitride layer is in a range of 1.9 to 2.5, and a thickness of the at least one silicon nitride layer is in a range of 50 nm to 100 nm. 
     In some embodiments, where forming the second passivation layer includes: depositing the at least one aluminum oxide layer on the rear surface of the substrate, wherein precursors of the at least one aluminum oxide layer include argon, trimethylaluminium and nitrous oxide, wherein a gas flow ratio of the argon, the trimethylaluminium and the nitrous oxide is in a range of 1:1:1 to 1.5:1:2. 
     In some embodiments, forming the PPW layer includes: depositing the at least one silicon oxynitride layer on a surface of the at least one aluminum oxide layer, wherein precursors of the at least one silicon oxynitride include silanes, ammonia and nitrous oxide, wherein a gas flow ratio of the silanes, the ammonia and the nitrous oxide is in a range of 1:1:3 to 1:4:6. 
     In some embodiments, forming the PPW layer includes: depositing an intermediate silicon oxide layer on a surface of the at least one aluminum oxide layer first, wherein precursors of the intermediate silicon oxide layer include silanes and nitrous oxide, wherein a gas flow ratio of the silanes and the nitrous oxide is in a range of 1:3 to 1:6; and introducing nitrogen source gas to produce nitrogen plasmas to react with the intermediate silicon oxide layer, so as to form the at least one silicon oxynitride layer. 
     In some embodiments, forming the at least one silicon nitride layer includes: depositing the at least one silicon nitride layer on a surface of the silicon oxynitride layer, wherein precursors of the at least one silicon nitride layer include the silanes and ammonia, wherein a gas flow ratio of the silanes and the ammonia is in a range of 1:1.3 to 1:4. 
     In some embodiments, the at least one silicon nitride layer includes three silicon nitride layers, and forming the three silicon nitride layers includes: forming a first silicon nitride layer on the surface of the silicon oxynitride layer, wherein precursors of the first silicon nitride layer include the silanes and the ammonia, wherein the gas flow ratio of the silanes and the ammonia is in a range of 1:1.3 to 1:1.5; forming a second silicon nitride layer on a surface of the first silicon nitride layer, wherein precursors of the second silicon nitride layer include the silanes and the ammonia, wherein the gas flow ratio of the silanes and the ammonia is in a range of 1:1.5 to 1:2.2; and forming a third silicon nitride layer on a surface of the second silicon nitride layer, wherein precursors of the third silicon nitride layer include the silanes and the ammonia, wherein the gas flow ratio of the silanes and the ammonia is in a range of 1:2.2 to 1:4. 
     In some embodiments, a thickness of the first silicon nitride layer is in a range of 5 nm to 20 nm, a thickness of the second silicon nitride layer is in a range of 20 nm to 40 nm, and a thickness of the third silicon nitride layer is in a range of 40 nm to 75 nm. 
     In some embodiments, a refractive index of the first silicon nitride layer is in a range of 2.1 to 2.5, a refractive index of the second silicon nitride layer is in a range of 2 to 2.3, and a refractive index of the third silicon nitride layer is in a range of 1.9 to 2.1. 
     In some embodiments, forming the second passivation layer further includes: forming a silicon oxide layer between the substrate and the at least one aluminum oxide layer. 
     In some embodiments, a thickness of the silicon oxide layer is in a range of 0.1 nm to 5 nm. 
     Embodiments of the present disclosure further provide photovoltaic module, including at least one photovoltaic cell string, wherein each of the at least one photovoltaic cell string is composed of a plurality of photovoltaic cells electrically connected; wherein each of the plurality of photovoltaic cells includes: a substrate; a first passivation layer and a first anti-reflection layer that are sequentially disposed on a front surface of the substrate in a direction away from the substrate; and a second passivation layer, a polarization phenomenon weakening (PPW) layer and at least one silicon nitride layer Si u N v  that are sequentially disposed on a rear surface of the substrate in a direction away from the substrate, wherein 1&lt;u/v&lt;4; wherein the second passivation layer includes at least one aluminum oxide layer Al x O y , wherein 0.8&lt;y/x&lt;1.6 and a refractive index of the at least one aluminum oxide layer is in a range of 1.4 to 1.6, and a thickness of the at least one aluminum oxide layer is in a range of 4 nm to 20 nm; wherein the PPW layer includes at least one silicon oxynitride layer Si r O s N t , wherein r&gt;s&gt;t and a refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8, and a thickness of the at least one silicon oxynitride layer is in a range of 1 nm to 30 nm; wherein a refractive index of the at least one silicon nitride layer is in a range of 1.9 to 2.5, and a thickness of the at least one silicon nitride layer is in a range of 50 nm to 100 nm. 
     In some embodiments, the at least one silicon nitride layer includes a first silicon nitride layer, a second silicon nitride layer and a third silicon nitride layer stacked in the direction away from the substrate, wherein a thickness of the first silicon nitride layer is in a range of 5 nm to 20 nm, a thickness of the second silicon nitride layer is in a range of 20 nm to 40 nm, and a thickness of the third silicon nitride layer is in a range of 40 nm to 75 nm. 
     In some embodiments, refractive indexes of the first silicon nitride layer, the second silicon nitride layer and the third silicon nitride layer decrease layer by layer in the direction away from the substrate, a refractive index of the first silicon nitride layer is in a range of between 2.1 to 2.5, a refractive index of the second silicon nitride layer is in a range of 2 to 2.3, and a refractive index of the third silicon nitride layer is in a range of 1.9 to 2.1. 
     In some embodiments, a concentration of silicon atoms in the at least one silicon oxynitride layer is between 5×10 21 /cm 3  and 2.5×10 22 /cm 3 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments are described as examples with reference to the corresponding figures in the accompanying drawings, and the examples do not constitute a limitation to the embodiments. Elements with the same reference numerals in the accompanying drawings represent similar elements. The figures in the accompanying drawings do not constitute a proportion limitation unless otherwise stated. 
         FIG.  1    is a schematic structural diagram of a photovoltaic cell according to some embodiments of the present disclosure. 
         FIG.  2    is another structural diagram of the photovoltaic cell according to some embodiments of the present disclosure. 
         FIG.  3    is still another structural schematic diagram of the photovoltaic cell according to some embodiments of the present disclosure. 
         FIG.  4    is a schematic flow chart of a method for manufacturing a photovoltaic cell according to some embodiments of the present disclosure. 
         FIG.  5    is a schematic structural diagram of a photovoltaic cell according to a comparative example provided in the present disclosure. 
         FIG.  6    is yet another schematic structural diagram of a photovoltaic cell according to some embodiments of the present disclosure. 
         FIG.  7    is still yet another schematic structural diagram of a photovoltaic cell according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings in order to make the objectives, technical solutions and advantages of the present disclosure clearer. However, it will be appreciated by those of ordinary skill in the art that, in various embodiments of the present disclosure, numerous technical details are set forth in order to provide the reader with a better understanding of the present disclosure. However, the technical solutions claimed in the present disclosure may be implemented without these technical details and various changes and modifications based on the following embodiments. 
     The present disclosure provides a photovoltaic cell, which includes a substrate; a first passivation layer and a first anti-reflection layer that are sequentially disposed on a front surface of the substrate in a direction away from the substrate; and a second passivation layer, a polarization phenomenon weakening (PPW) layer and at least one silicon nitride layer Si u N v  that are sequentially disposed on a rear surface of the substrate in a direction away from the substrate, where 1&lt;u/v&lt;4. The second passivation layer includes at least one aluminum oxide layer Al x O y , where 0.8&lt;y/x&lt;1.6 and a refractive index of the at least one aluminum oxide layer is in a range of 1.4 to 1.6, and a thickness of the at least one aluminum oxide layer is in a range of 4 nm to 20 nm. The PPW layer includes at least one silicon oxynitride layer Si r O s N t , where r&gt;s&gt;t and a refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8, and a thickness of the at least one silicon oxynitride layer is in a range of 1 nm to 30 nm. A refractive index of the at least one silicon nitride layer is in a range of 1.9 to 2.5, and a thickness of the at least one silicon nitride layer is in a range of 50 nm to 100 nm. 
     By disposing the PPW layer including the at least one silicon oxynitride layer between the at least one aluminum oxide layer and the at least one silicon nitride layer, a potential difference between the at least one aluminum oxide layer and the at least one silicon nitride layer may be reduced, and the anti-PID performance of the photovoltaic cell may be improved, thus ensuring the high conversion efficiency of the photovoltaic cell. Furthermore, a refractive index of each layer on the rear surface of the photovoltaic cell is within a reasonable refractive index range by defining a relationship between the atom number of each kind of atoms in the silicon nitride layer, the aluminum oxide layer and the silicon oxynitride layer. When the refractive index of all the layers on the rear surface of the photovoltaic cell is within the reasonable refractive index range and each layer has a suitable thickness, the light utilization rate of the photovoltaic cell can be increased and the light conversion efficiency of the photovoltaic cell can be improved. 
     Some embodiments of the photovoltaic cell of the present disclosure will be described in detail below. The following contents are merely provided for convenience of understanding the implementation details, and are not necessary for the implementation of the technical solution of the present disclosure. 
       FIG.  1    is a schematic structural diagram of a photovoltaic cell according to some embodiments of the present disclosure. 
     As shown in  FIG.  1   , the photovoltaic cell includes a substrate  10 . The substrate  10  includes an intrinsic silicon substrate (or a silicon substrate)  11  and an emitter  12 . The intrinsic silicon substrate  11  and the emitter  12  form a PN junction of the photovoltaic cell. For example, the intrinsic silicon substrate  11  may be a P-type substrate, the emitter  12  may be an N-type doped layer, that is, the P-type substrate and the N-type doped layer form a PN junction. As another example, the intrinsic silicon substrate  11  may be an N-type substrate, the emitter  12  may be a P-type doped layer, that is, the N-type substrate and the P-type doped layer form a PN junction. In some embodiments, the intrinsic silicon substrate  11  includes, but is not limited to, a monocrystalline silicon substrate, a polycrystalline silicon substrate, a monocrystalline silicon-like substrate, etc. It should be noted that a front surface of the substrate  10  is designated as a light-receiving surface, and a rear surface of the substrate  10  refers to a surface opposite to the front surface. In some embodiments, a surface close to the emitter  12  is referred to as the front surface, and a surface close to the intrinsic silicon substrate  11  is referred to as the rear surface. 
     In some embodiments, the emitter  12  and the intrinsic silicon substrate  11  are formed in one original silicon wafer. In some embodiments, the emitter  12  is an additional layer disposed over the front surface of the intrinsic silicon substrate  11 . In some embodiments, the emitter  12  is omitted. 
     The photovoltaic cell further includes a first passivation layer  13  and a first anti-reflection layer  14  that are sequentially disposed on the front surface of the substrate  10  in a direction away from the substrate  10 . In some embodiments, the photovoltaic cell further includes a first electrode  15  penetrating through the first passivation layer  13  and the first anti-reflection layer  14  and forming an ohmic contact with the emitter  12  of the substrate  10 . 
     The first passivation layer  13  includes, but is not limited to, a silicon oxide layer, an aluminum oxide layer, a silicon nitride layer, a silicon oxynitride layer, etc. The first passivation layer  13  is configured to reduce the recombination of carriers, thereby increasing an open circuit voltage and a short circuit current of the photovoltaic cell. The first anti-reflection layer  14  may be provided with a layer similar to or substantially the same as the first passivation layer  13 , for example, including but not limited to, the silicon oxide layer, the aluminum oxide layer, the silicon nitride layer, the silicon oxynitride layer, etc. The first anti-reflection layer  14  may not only reduce the reflectivity of lights incident on a surface of the photovoltaic cell, but also passivate the surface of the photovoltaic cell. For those skilled in the art, one or both of the first passivation layer  13  and the first anti-reflection layer  14  may be formed by a PECVD process, and one or both of the first passivation layer  13  and the first anti-reflection layer  14  may include hydrogen elements due to the PECVD process. 
     In some embodiments, the first passivation layer  13  may be a single layer or a multi-layer structure, or the first anti-reflection layer  14  may be a single layer or a multi-layer structure. For example, the first passivation layer  13  may include a silicon oxide layer, the first anti-reflection layer  14  may include a silicon nitride sub-layer and a silicon oxynitride sub-layer. As another example, the first passivation layer  13  may include a silicon oxide layer, the first anti-reflection layer  14  may include a silicon nitride sub-layer, a silicon oxynitride sub-layer and a silicon oxide sub-layer. 
     The photovoltaic cell further includes a second passivation layer  20 , a polarization phenomenon weakening (PPW) layer  18  and a silicon nitride layer  19  that are sequentially disposed on the rear surface of the substrate  10  in a direction away from the substrate  10 . In some embodiments, the photovoltaic cell further includes a second electrode  21  penetrating through the second passivation layer  20 , the polarization phenomenon weakening layer  18  and the silicon nitride layer  19  and forming an ohmic contact with the substrate  10 . 
     In some embodiments, the second passivation layer  20  includes at least one aluminum oxide layer Al x O y , where 0.8&lt;y/x&lt;1.6. Particularly, 0.8&lt;y/x&lt;1, 1&lt;y/x&lt;1.5, or 1.5&lt;y/x&lt;1.6. As used herein, x and y denote the number of aluminum atoms and the number of oxygen atoms, respectively, in the at least one aluminum oxide layer. When the at least one aluminum oxide layer is provided with a single layer (i.e., an aluminum oxide layer  17  shown in  FIG.  1   ), a thickness of the aluminum oxide layer  17  is in a range of 4 nm to 20 nm. For example, the thickness of the aluminum oxide layer  17  is 5 nm, 10 nm, 15 nm or 20 nm. When forming the aluminum oxide layer  17  with a particular thickness, a ratio of y and x in the aluminum oxide layer  17  is controlled in a range of 0.8 to 1.6, and a refractive index of the aluminum oxide layer  17  is in a range of 1.4 to 1.65, such as 1.4-1.6. Particularly, the refractive index of the aluminum oxide layer  17  is in a range of 1.55 to 1.59. It should be noted that, when the at least one aluminum oxide layer is provided with a plurality of layers (not shown), the refractive index mentioned here should be a refractive index of all the aluminum oxide layers, that is, the refractive index of all of the plurality of aluminum oxide layers is in a range of 1.4 to 1.65. Particularly, the refractive index of all of the plurality of aluminum oxide layers is in a range of 1.55 to 1.59. 
     As shown in  FIG.  1   , the second passivation layer  20  further includes a silicon oxide layer  16 . The silicon oxide layer  16  is disposed between the substrate  10  and the aluminum oxide layer  17  to isolate the aluminum oxide layer  17  from the substrate  10 , thereby avoiding a direct contact between the aluminum oxide layer  17  and the substrate  10 . A dense silicon oxide layer  16  is chemically stable, which may chemically passivate a dangling bond on the surface of the substrate  10 . Herein, a thickness of the silicon oxide layer  16  is in a range of 0.1 nm to 5 nm. Particularly, the thickness of the silicon oxide layer  16  is 2 nm, 3 nm or 4 nm. 
     It is found through an experimental verification that the PID may be improved by 21.2% to 27.7% via providing the silicon oxide layer  16  with a special design (for example, the thickness of the silicon oxide layer  16  is in a range of 0.1 nm to 5 nm) compared with providing a passivation layer without silicon oxide. 
     The polarization phenomenon weakening layer  18  includes at least one silicon oxynitride layer Si r O s N t , where r&gt;s&gt;t. As used herein, r, s and t denote the number of silicon atoms, the number of oxygen atoms and the number of nitride atoms, respectively, in the at least one silicon oxynitride layer. A concentration of silicon atoms in the at least one silicon oxynitride layer is in a range of 5×10 21 /cm 3  to 2.5×10 22 /cm 3 . The PPW layer  18  is configured to reduce a cell difference between layers on two sides of the PPW layer  18 , so as to improve the anti-PID effect. In some embodiments, a thickness of the at least one silicon oxynitride layer in the PPW layer  18  is in a range of 1 nm to 30 nm. Particularly, the thickness of the at least one silicon oxynitride layer is 6 nm, 11 nm, 16 nm, 21 nm, 25 nm or 30 nm. When forming the at least one silicon oxynitride layer Si r O s N t  with a particular thickness, where r&gt;s&gt;t, a refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8. It should be noted that when the at least one silicon oxynitride layer is provided with a plurality of layers (not shown), the refractive index mentioned here should be a refractive index of all the silicon oxynitride layers, that is, the refractive index of all of the plurality of silicon oxynitride layers is in a range of 1.5 to 1.8. 
     In some embodiments, as shown in  FIGS.  6  and  7   , the PPW layer  18  includes a thin silicon oxide layer  18   a  (referred to as a second silicon oxide layer  18   a  different from the silicon oxide layer  16 ) and at least one silicon oxynitride layer  18   b . The second silicon oxide layer  18   a  is disposed between the aluminum oxide layer  17  and the at least one silicon oxynitride layer  18   b . In some embodiments, the second silicon oxynitride layer  18   a  of the PPW layer  18  is formed by depositing an intermediate silicon oxide layer, thus the thin silicon oxide layer  18   a  is formed between the aluminum oxide layer  17  and the silicon oxynitride layer  18   b . A thickness of the thin silicon oxide layer  18   a  is in a range of 0.5 nm to 5 nm. In some embodiments, the thin silicon oxide layer  18   a  is formed by an additional oxidation process. 
     It is found through an experimental verification that the PID may be improved by 42.8% to 69.60% via providing the PPW layer  18  with a specific design (for example, a thickness of the PPW layer is in a range of 1 nm to 30 nm) compared with providing a passivation layer without the PPW layer. 
     It is found through an experimental verification that, in some embodiments, the PID may be improved by up to 81.6% to 99.00% when the rear surface of the photovoltaic cell is provided with both the silicon oxide layer  16  and the PPW layer  18  for isolation. 
     In an embodiment, the at least one silicon nitride layer Si u N v  is provided with a single layer (i.e., the silicon nitride layer  19  shown in  FIG.  1   ), where 1&lt;u/v&lt;4. Particularly, 1&lt;u/v&lt;2 or 2&lt;u/v&lt;4. As used herein, u and v denote the number of silicon atoms and the number of nitride atoms, respectively, in the at least one silicon nitride layer. A thickness of the silicon nitride layer  19  is in a range of 50 nm to 100 nm. Particularly, the thickness of the silicon nitride layer  19  is 60 nm, 75 nm or 90 nm. When forming the silicon nitride layer  19  with a particular thickness, the ratio of u and v in the silicon nitride layer  19  may be controlled in a range of 1 to 4, and a refractive index of the silicon nitride layer  19  is in a range of 1.9 to 2.5. In some embodiments, the at least one silicon nitride layer is provided with a plurality of layers (shown in  FIG.  2    and  FIG.  3   ), for example, 2 to 5 layers. Refractive indexes of the plurality of silicon nitride layers decrease layer by layer in the direction away from the substrate  10 , but a refractive index of all the silicon nitride layers should be controlled in a range of 1.9 to 2.5. It should be noted that the number of the plurality of silicon nitride layers may be configured according to requirements on the thickness and refractive index of the deposited silicon nitride layer, which is not limited in the present disclosure. 
     In some embodiments, as shown in  FIG.  7   , a third silicon oxide layer  22  is disposed on a surface of the silicon nitride layer  19  facing away from the rear surface of the substrate  10 . A thickness of the third silicon oxide layer  22  is in a range of 0.5 nm to 5 nm. The number of silicon atoms is not less than the number of oxygen atoms in the third silicon oxide layer  22 . In some embodiments, the third silicon oxide layer  22  is omitted. 
     In order to achieve a photovoltaic cell with high anti-PID effect and high efficiency, for example, the thicknesses of the second passivation layer  20 , the PPW layer  18  and the silicon nitride layer  19  on the rear surface of the photovoltaic cell and their corresponding refractive indexes are designed to be matched. The refractive index of all the layers on the rear surface of the photovoltaic cell is within a reasonable refractive index range by defining a relationship of the atom number of each kind of atoms in the aluminum oxide layer  17  included in the second passivation layer  20 , the polarization phenomenon weakening layer  18  and the silicon nitride layer  19 . When the refractive index of all the layers on the rear surface of the photovoltaic cell is within the reasonable refractive index range and each layer has a suitable thickness, which result in a relatively high anti-reflective property, the light utilization rate of the photovoltaic cell can be increased and the light conversion efficiency of the photovoltaic cell can be improved. 
     In some embodiments of the present disclosure, the aluminum oxide layer  17  is provided on the rear surface of the photovoltaic cell. Since the growth and annealing temperature of the aluminum oxide layer  17  is relatively low, octahedral structures of aluminum atoms in the aluminum oxide layer  17  will be transformed into tetrahedral structures after a high temperature heat treatment to generate interstitial oxygen atoms. The interstitial oxygen atoms capture valence electrons in the substrate  10  to form fixed negative charges, so that the aluminum oxide layer  17  shows an electronegativity and an interface electric field directed to the inside of the substrate  10  is generated at the interface, thus causing carriers to escape from the interface quickly, reducing an interface recombination rate and increasing a minority carrier lifetime of the substrate  10 . The PPW layer  18  disposed on the aluminum oxide layer  17  may effectively prevent subsequent products of sodium ions, ˜OH and ˜CH3 groups from migrating into the photovoltaic cell, block the movement and migration of mobile ions under an external electric field, temperature and humidity, and reduce the potential difference between layers and enhance the anti-PID effect, thus having better anti-PID performance and anti-aging/attenuation performance. The silicon nitride layer  19  disposed on the PPW layer  18  achieves the optimal anti-reflection effect by combining optical path matching, and protects the adjacent aluminum oxide layer  17  and polarization phenomenon weakening layer  18  from corrosion caused by the excessive paste. After annealing, an H passivation effect of the silicon nitride layer  19  is significant, which further improves the minority carrier lifetime of a silicon wafer and also prevents subsequent products of Na+, ˜OH and ˜CH3 groups from migrating into the photovoltaic cell to a certain extent, thus avoiding power attenuation caused by electric leakage of cell components. The combination of the aluminum oxide layer  17  and the polarization phenomenon weakening layer  18  reduces the power loss of the cell components, and improves light attenuation performance, heat-assisted light attenuation performance and anti-PID performance of the photovoltaic cell. 
     In an embodiment, as shown in  FIG.  2   , the at least one silicon nitride layer is provided with three silicon nitride layers, i.e., a first silicon nitride layer  191 , a second silicon nitride layer  192  and a third silicon nitride layer  193  that are stacked in the direction away from the substrate  10 . In this embodiment, a refractive index of the three and silicon nitride layers is in a range of 1.9 to 2.5. A thickness of the first silicon nitride layer  191  is in a range of 5 nm to 20 nm, a thickness of the second silicon nitride layer  192  is in a range of 20 nm to 40 nm, and a thickness of the third silicon nitride layer  193  is in a range of 40 nm to 75 nm. A refractive index of the first silicon nitride layer  191  is in a range of 2.1 to 2.5, a refractive index of the second silicon nitride layer  192  is in a range of 2 to 2.3, and a refractive index of the third silicon nitride layer  193  is in a range of 1.9 to 2.1. It should be noted that although there are the same values in the refractive index ranges of every two silicon nitride layers in the above three silicon nitride layers, the refractive indexes of the three silicon nitride layers need to satisfy the condition that “the refractive indexes of the plurality of silicon nitride layers decrease layer by layer in the direction away from the substrate  10 ” in practical applications. Therefore, a situation that every two silicon nitride layers in the three silicon nitride layers have the same refractive indexes may not happen. 
     In an embodiment, as shown in  FIG.  3   , the at least one silicon nitride layer is provided with two silicon nitride layers, i.e., a first silicon nitride layer  191  and a second silicon nitride layer  192  that are stacked in the direction away from the substrate  10 . In this embodiment, a refractive index of the two silicon nitride layers is in a range of 1.9 to 2.5. A thickness of the first silicon nitride layer  191  is in a range of 15 nm to 40 nm, and a thickness of the second silicon nitride layer  192  is in a range of 35 nm to 110 nm. A refractive index of the first silicon nitride layer  191  is in a range of 2.3 to 2.5, and a refractive index of the second silicon nitride layer  192  is in a range of 1.9 to 2.2. 
     Some embodiments of the present disclosure provide a photovoltaic module, which includes at least one photovoltaic cell string. The photovoltaic cell string is composed of the above photovoltaic cells electrically connected, for example, the photovoltaic cells illustrated in  FIGS.  1  to  3   . The photovoltaic cells are electrically connected in series and/or parallel in the photovoltaic cell string. The photovoltaic module includes, but is not limited to, a laminate module, a double-sided module, a multi-main grid module, etc. For example, the photovoltaic cells (e.g.,  FIG.  1   ) described above can be obtained, and the cells can be electrically connected with adjacent ones via conductive materials to form the cell string. A back plate, an ethylene-vinyl acetate (EVA) copolymer and the cell string are stacked in a certain order through a lamination process. Then the stacked structure is installed with a frame to form the photovoltaic module. The photovoltaic cells may convert absorbed light energy into electric energy. The module may transfer the electric energy obtained by the cells to a load. 
     Some embodiments of the present disclosure provide a method for manufacturing the photovoltaic cell described in the above embodiments. A schematic flow chart of the method for manufacturing the photovoltaic cell is shown in  FIG.  4   , which includes the following steps. 
     In step  101 , a substrate is provided. 
     Specifically, the substrate includes an intrinsic silicon substrate and an emitter. The intrinsic silicon substrate and the emitter form a PN junction of the photovoltaic cell. As shown in  FIGS.  1  to  3   , the substrate  10  includes an intrinsic silicon substrate  11  and an emitter  12 . The intrinsic silicon substrate  11  and the emitter  12  form a PN junction. For example, the intrinsic silicon substrate  11  may be a P-type substrate, the emitter  12  may be an N-type doped layer, that is, the P-type substrate and the N-type doped layer form a PN junction. In some embodiments, the intrinsic silicon substrate  11  includes, but is not limited to, a monocrystalline silicon substrate, a polycrystalline silicon substrate, a monocrystalline silicon-like substrate, etc. It should be noted that a front surface of the substrate  10  is designated as a light-receiving surface, and a rear surface of the substrate  10  refers to a surface opposite to the front surface. In some embodiments, a surface close to the emitter  12  is referred to as the front surface, and a surface close to the intrinsic silicon substrate  11  is referred to as the rear surface. 
     In step  102 , a first passivation layer, a first anti-reflection layer and a first electrode are sequentially disposed on a front surface of the substrate in a direction away from the substrate. 
     As shown in  FIGS.  1  to  3   , a first passivation layer  13  and a first anti-reflection layer  14  are sequentially stacked on the front surface of the substrate  10  in the direction away from the substrate  10 . Herein, the first passivation layer  13  includes, but is not limited to, an aluminum oxide layer, a silicon nitride layer, a silicon oxynitride layer, etc. The first passivation layer  13  is used to reduce the recombination of carriers, thereby increasing an open circuit voltage and a short circuit current of the photovoltaic cell. The first anti-reflection layer  14  may be provided with a layer similar to or substantially the same as the passivation layer  13 , for example, including but not limited to, the aluminum oxide layer, the silicon nitride layer, the silicon oxynitride layer, etc. The first anti-reflection layer  14  may not only reduce the reflectivity of lights incident on a surface of the photovoltaic cell, but also passivate the surface of the photovoltaic cell. 
     The first passivation layer  13  or the first anti-reflection layer  14  may be formed by, including but not limited to, plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapour deposition (PVD), etc. 
     In an embodiment, a first electrode  15  is further formed on the front surface of the substrate  10 . The first electrode  15  penetrates through the first passivation layer  13  and the first anti-reflection layer  14 , and forms an ohmic contact with the emitter  12  of the substrate  10 . The first electrode  15  may be formed by a metallization process, for example, by screen printing the conductive paste. 
     In step  103 , a second passivation layer is formed on a rear surface of the substrate in the direction away from the substrate. 
     As shown in  FIGS.  1  to  3   , a second passivation layer  20  is formed on the rear surface of the substrate  10 . The second passivation layer  20  includes at least one aluminum oxide layer Al x O y , where 0.8&lt;y/x&lt;1.6. Particularly, 0.8&lt;y/x&lt;1, 1&lt;y/x&lt;1.5 or 1.5&lt;y/x&lt;1.6. When the at least one aluminum oxide layer is provided with a single layer (i.e., an aluminum oxide layer  17 ), a thickness of the aluminum oxide layer  17  is in a range of 4 nm to 20 nm. Particularly, the thickness of the aluminum oxide layer  17  is 5 nm, 10 nm, 15 nm or 20 nm. When forming the aluminum oxide layer  17  with a particular thickness, a ratio of y and x in the aluminum oxide layer  17  is controlled in a range of 0.8 to 1.6, and a refractive index of the aluminum oxide layer  17  is in a range of 1.4 to 1.6. Particularly, the refractive index of the aluminum oxide layer  17  is in a range of 1.55 to 1.59. It should be noted that when the at least one aluminum oxide layer is provided with a plurality of layers (not shown), the refractive index mentioned here should be a refractive index of all the aluminum oxide layers, that is, the refractive index of all of the plurality of aluminum oxide layers is in a range of 1.4 to 1.6. Particularly, the refractive index of all of the plurality of aluminum oxide layers is in a range of 1.55 to 1.59. 
     In an embodiment, the aluminum oxide layer  17  in the second passivation layer  20  is prepared by the PECVD. Argon, trimethylaluminium and nitrous oxide may be used as precursors of the aluminum oxide layer  17 . Herein, a gas flow ratio of the argon, the trimethylaluminium and the nitrous oxide is in a range of 1:1:1 to 1.5:1:2. Particularly, the gas flow ratio is in a range of 1:1:1 to 1:1:2, and the pressure in a PECVD reaction chamber is 0.13 mbar. A thickness of the aluminum oxide layer  17  is in a range of 4 nm to 20 nm. A ratio of y and x in the aluminum oxide layer  17  may be controlled in a range of 0.8 to 1.6. A refractive index of the aluminum oxide layer  17  is in a range of 1.4 to 1.65. 
     In an embodiment, the second passivation layer  20  further includes a silicon oxide layer  16 . The silicon oxide layer  16  is disposed between the substrate  10  and the aluminum oxide layer  17 . The silicon oxide layer  16  is formed between the substrate  10  and the aluminum oxide layer  17  to isolate the aluminum oxide layer  17  from the substrate  10 . The silicon oxide layer  16  is formed by applying an ozone (O 3 ) process in a process of etching the substrate  10 . The dense silicon oxide layer  16  is chemically stable, which may chemically passivate a dangling bond on the surface of the substrate  10 . A thickness of the silicon oxide layer  16  is in a range of 0.1 nm to 5 nm. Particularly, the thickness of the silicon oxide layer  16  is 2 nm, 3 nm or 4 nm. 
     In step  104 , a polarization phenomenon weakening (PPW) layer is formed on a surface of the second passivation layer away from the substrate. 
     The PPW layer may be configured as an intermediate layer to reduce a potential difference between its upper and lower layers, thus improving the anti-PID performance of the photovoltaic cell and further ensuring the high conversion efficiency of the photovoltaic cell. In some embodiments, as shown in  FIGS.  1  to  3   , the PPW layer  18  includes at least one silicon oxynitride layer Si r O s N t , where r&gt;s&gt;t. A concentration of silicon atoms in the at least one silicon oxynitride layer is in a range of 5×10 21 /cm 3  to 2.5×10 22 /cm 3 . 
     In an embodiment, when depositing the at least one silicon oxynitride layer on the surface of the aluminum oxide layer  17 , silanes, ammonia and nitrous oxide are simultaneously introduced into a reaction chamber, where a gas flow ratio of the silanes, the ammonia and the nitrous oxide is in a range of 1:1:3 to 1:4:6, and the pressure in the reaction chamber is 0.25 mbar. A thickness of the at least one silicon oxynitride layer is in a range of 1 nm to 30 nm, and a refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8. It should be noted that when the at least one silicon oxynitride layer is provided with a plurality of layers, the refractive index mentioned here should be a refractive index of all the silicon oxynitride layers, that is, the refractive index of all of the plurality of silicon oxynitride layers is in a range of 1.5 to 1.8. 
     In an embodiment, an intermediate silicon oxide layer is deposited on the surface of the aluminum oxide layer  17 , and precursors of the intermediate silicon oxide layer are the silanes and the nitrous oxide, where a gas flow ratio of the silanes and the nitrous oxide is in a range of 1:3 to 1:6, and the pressure in the reaction chamber is 0.25 mbar. After the intermediate silicon oxide layer is formed, nitrogen source gas is introduced to produce nitrogen plasmas to react with the intermediate silicon oxide layer, so as to form the at least one silicon oxynitride layer. That is to say, an intermediate silicon dioxide layer is formed first by the reaction on the surface of the aluminum oxide layer  17 , and then the nitrogen source gas is introduced to produce nitrogen plasmas to react with the silicon dioxide layer, so as to form the at least one silicon oxynitride layer. A thickness of the at least one silicon oxynitride layer is in a range of 1 nm to 30 nm, and the refractive index of the at least one silicon oxynitride layer is in a range of 1.5 to 1.8. 
     In step  105 , at least one silicon nitride layer is formed on a surface of the polarization phenomenon weakening layer away from the substrate. 
     As shown in  FIG.  1   , the at least one silicon nitride layer Si u N v  is provided with a single layer (i.e., the silicon nitride layer  19 ), where 1&lt;u/v&lt;4, and the silicon nitride layer  19  is formed on the surface of the PPW layer  18 . A thickness of the silicon nitride layer  19  is in a range of 50 nm to 100 nm. 
     In an embodiment, the silicon nitride layer  19  is prepared by the PECVD, the silanes and the ammonia may be used as precursors of the silicon nitride layer  19 . The pressure in the reaction chamber is 0.25 mbar, and a gas flow ratio of the silanes and the ammonia is in a range of 1:1.3 to 1:4. A thickness of the silicon nitride layer  19  is in a range of 50 nm to 100 nm, and a refractive index of the silicon nitride layer  19  is in a range of 1.9 to 2.5. Particularly, the thickness of the silicon nitride layer  19  is 60 nm, 75 nm or 90 nm. 
     In an embodiment, the at least one silicon nitride layer is provided with a plurality of silicon nitride layers. Particularly, the at least one silicon nitride layer is provided with 2 to 5 silicon nitride layers, such as 2 layers, 3 layers, etc. As shown in  FIG.  2   , the at least one silicon nitride layer includes a first silicon nitride layer  191 , a second silicon nitride layer  192  and a third silicon nitride layer  193 . Specifically, precursors of the three silicon nitride layers are introduced into a first reaction chamber of a PECVD equipment, and the precursors are the silanes and the ammonia. A gas flow ratio of the silanes and the ammonia is in a range of 1:1.3 to 1:1.5, the pressure in the first reaction chamber is 0.25 mbar, and the first silicon nitride layer  191  is formed by the PECVD process. The same kind of precursors are continuously introduced into the first reaction chamber, a gas flow ratio of the silanes and the ammonia is in a range of 1:1.5 to 1:2.2, the pressure in the first reaction chamber is 0.25 mbar, and the second silicon nitride layer  192  is formed by the PECVD process. The same kind of precursors are introduced into a second reaction chamber of the PECVD equipment, a gas flow ratio of the silanes and the ammonia is in a range of 1:2.2 to 1:4, the pressure in the reaction chamber is 0.25 mbar, and the third silicon nitride layer  193  is formed by the PECVD process. 
     Based on the preparation processes, a thickness of the first silicon nitride layer  191  is in a range of 5 nm to 20 nm, a thickness of the second silicon nitride layer  192  is in a range of 20 nm to 40 nm, and a thickness of the third silicon nitride layer  193  is in a range of 40 nm to 75 nm. A refractive index of the first silicon nitride layer  191  is in a range of 2.1 to 2.5, a refractive index of the second silicon nitride layer  192  is in a range of 2 to 2.3, and a refractive index of the third silicon nitride layer  193  is in a range of 1.9 to 2.1. A refractive index of the three silicon nitride layers is in a range of 1.9 to 2.5. It should be noted that although there are the same values in the refractive index ranges of every two silicon nitride layers in the above three silicon nitride layers, the refractive indexes of the three silicon nitride layers need to satisfy the condition that “the refractive indexes of the plurality of silicon nitride layers decrease layer by layer in the direction away from the substrate  10 ” in practical applications. Therefore, a situation that every two silicon nitride layers in the three silicon nitride layers have the same refractive indexes may not happen. 
     In an embodiment, as shown in  FIG.  3   , the at least one silicon nitride layer is provided with two silicon nitride layers, i.e., a first silicon nitride layer  191  and a second silicon nitride layer  192 . Specifically, precursors of the two silicon nitride layers are introduced into a first reaction chamber of a PECVD equipment, and the reactants are the silanes and the ammonia. A gas flow ratio of the silanes and the ammonia is 1:1.9, a reaction chamber pressure is 0.25 mbar, and the first silicon nitride layer  191  is formed by the PECVD process. The same kind of precursors are continuously introduced into the first reaction chamber, a gas flow ratio of the silanes and the ammonia is 1:2.8, a reaction chamber pressure is 0.25 mbar, and the second silicon nitride layer  192  is formed by the PECVD process. 
     Based on the above preparation processes, a thickness of the first silicon nitride layer  191  is in a range of 15 nm to 40 nm, and a thickness of the second silicon nitride layer  192  is in a range of 35 nm to 110 nm. A refractive index of the first silicon nitride layer  191  is in a range of 2.3 to 2.5, and a refractive index of the second silicon nitride layer  192  is in a range of 1.9 to 2.2. 
     In step  106 , a conductive paste is printed on the surface of the at least one silicon nitride layer and sintered to form a second electrode. The second electrode penetrates through the second passivation layer, the PPW layer and the at least one silicon nitride layer, and forms an ohmic contact with the substrate. 
     In order to achieve a photovoltaic cell with high anti-PID effect and high efficiency, for example, the thicknesses of the second passivation layer  20 , the PPW layer  18  and the silicon nitride layer  19  on the rear surface of the photovoltaic cell and their corresponding refractive indexes are designed to be matched. A relationship of the atom number of each kind of atoms in the aluminum oxide layer  17  included in the second passivation layer  20 , the PPW layer  18  and the silicon nitride layer  19  is specified through a proper process, so that the refractive index of all the layers on the rear surface of the photovoltaic cell is within a reasonable refractive index range. When the refractive index of all the layers on the rear surface of the photovoltaic cell is within the reasonable refractive index range and each layer has a suitable thickness, which result in a relatively high anti-reflective property, the light utilization rate of the photovoltaic cell can be increased and the light conversion efficiency of the photovoltaic cell can be improved. 
     In some embodiments of the present disclosure, the aluminum oxide layer  17  is provided on the rear surface of the photovoltaic cell. Since the growth and annealing temperature of the aluminum oxide layer  17  is relatively low, octahedral structures of aluminum atoms in the aluminum oxide layer  17  will be transformed into tetrahedral structures after a high temperature heat treatment to generate interstitial oxygen atoms. The interstitial oxygen atoms capture valence electrons in the substrate  10  to form fixed negative charges, so that the aluminum oxide layer  17  shows an electronegativity and an interface electric field directed to the inside of the substrate  10  is generated at the interface, thus causing carriers to escape from the interface quickly, reducing an interface recombination rate and increasing a minority carrier lifetime of the substrate  10 . The PPW layer  18  disposed on the aluminum oxide layer  17  may effectively prevent subsequent products of sodium ions, ˜OH and ˜CH3 groups from migrating into the photovoltaic cell, block the movement and migration of mobile ions under an external electric field, temperature and humidity, and reduce the potential difference between layers and enhance the anti-PID effect, thus having better anti-PID performance and anti-aging/attenuation performance. The silicon nitride layer  19  disposed on the PPW layer  18  achieves the optimal anti-reflection effect by combining optical path matching, and protects the adjacent aluminum oxide layer  17  and polarization phenomenon weakening layer  18  from corrosion caused by the excessive paste. After annealing, an H passivation effect of the silicon nitride layer  19  is significant, which further improves the minority carrier lifetime of a silicon wafer and also prevents subsequent products of Na+, ˜OH and ˜CH3 groups from migrating into the photovoltaic cell to a certain extent, thus avoiding power attenuation caused by electric leakage of cell components. The combination of the aluminum oxide layer  17  and the polarization phenomenon weakening layer  18  reduces the power loss of the cell components, and improves light attenuation performance, heat-assisted light attenuation performance and anti-PID performance of the photovoltaic cell. 
     Comparative Example 
     A comparative example provides a back structure of a PERC cell. The specific structure is shown in  FIG.  5   , which includes: a substrate  10  having a PN junction; a first passivation layer  13 , a first anti-reflection layer  14  and a first electrode  15  that are sequentially disposed on a front surface of the substrate  10  in a direction away from the substrate  10 ; a second passivation layer  20 , a silicon nitride layer  19  Si u N v  and a second electrode  21  that are sequentially disposed on a rear surface of the substrate  10  in a direction away from the substrate  10 , where 1&lt;u/v&lt;4. The second passivation layer  20  includes at least one aluminum oxide layer Al x O y  (shown as an aluminum oxide layer  17  in  FIG.  5    when provided with a single layer), where 0.8&lt;y/x&lt;1.6. A refractive index of the at least one aluminum oxide layer is in a range of 1.4 to 1.6, and a thickness of the at least one aluminum oxide layer is in a range of 4 nm to 20 nm. A refractive index of the silicon nitride layer  19  is in a range of 1.9 to 2.5, and a thickness of the silicon nitride layer  19  is in a range of 50 nm to 100 nm. The second passivation layer  20  further includes a silicon oxide layer  16 . The silicon oxide layer  16  is disposed between the substrate  10  and the aluminum oxide layer  17  to isolate the aluminum oxide layer  17  from the substrate  10 , which may avoid a direct contact between the aluminum oxide layer  17  and the substrate  10 . 
     Compared with the photovoltaic cell in the present disclosure shown in  FIG.  1   , the difference is that the back structure of the comparative example does not have the PPW layer  18 , and other structures and preparation method are the same. Through a comparative experiment, the results are shown in the following table. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                   
                 Conversion 
                 Open circuit 
                 Short circuit 
                 Parallel 
                   
               
               
                   
                 efficiency 
                 voltage 
                 current 
                 resistance 
                 Fill factor 
               
               
                 Parameter 
                 Ncell/% 
                 Uoc/mV 
                 Isc/A 
                 Rs/mΩ 
                 FF/% 
               
               
                   
               
               
                 With Si r O s N t   
                 22.897 
                 688.2 
                 10.761 
                 0.988 
                 82.677 
               
               
                 Without Si r O s N t   
                 22.779 
                 685.6 
                 10.752 
                 0.955 
                 82.626 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Minority 
                 Minority 
               
               
                   
                 Thickness of 
                 carrier lifetime 
                 carrier lifetime 
               
               
                 Parameter 
                 layer 
                 (before sintering) 
                 (after sintering) 
               
               
                   
               
               
                 With Si r O s N t   
                 97 
                 128.71 
                 204.93 
               
               
                 Without Si r O s N t   
                 84.8 
                 93.91 
                 168.3 
               
               
                   
               
            
           
         
       
     
     Herein, the conversion efficiency of the photovoltaic cell=(open circuit voltage*short circuit current*fill factor)/(cell area*illumination amplitude)*100%. It can be seen that the open circuit voltage, the short circuit current and the fill factor are proportional to the conversion efficiency. The longer the minority carrier lifetime, the higher the conversion efficiency. It can be seen from the data in the table that a conversion efficiency of a photovoltaic cell with the Si r O s N t  on the rear surface is 0.118% higher than that of a photovoltaic cell without the Si r O s N t  on the rear surface. 
     The steps in the above methods only aim to make the description clearer. In implementation, the steps may be combined into one or one step may be divided into multiple sub-steps, which, as long as the same logical relationship is included, all fall into the protection scope of the present disclosure. Such a trivial amendment or design added to an algorithm or procedure as not changing the algorithm or a core design of the procedure falls into the protection scope of the disclosure. 
     It is not difficult to find that this embodiment is a method embodiment related to the first embodiment, and this embodiment may be implemented in cooperation with the first embodiment. The relevant technical details mentioned in the first embodiment are still valid in this embodiment, thus not repeated herein in order to reduce repetition. Accordingly, the relevant technical details mentioned in this embodiment may also be applied in the first embodiment. 
     Those skilled in the art should appreciate that the aforementioned embodiments are specific embodiments for implementing the present disclosure. In practice, however, various changes may be made in the forms and details of the specific embodiments without departing from the spirit and scope of the present disclosure.