Surface passivation process for improving conversion efficiency of back contact battery

Abstract

The invention discloses a surface passivation process for improving the conversion efficiency of a back contact battery. The surface passivation process comprises the following steps: 1) depositing an aluminum oxide film; 2) depositing a silicon nitride film; in the step 1), the deposition temperature of the aluminum oxide film is 200-260 DEG C. The high-compactness aluminum oxide film is obtained at the low aluminum oxide film deposition temperature, the requirement of the back contact battery for the thickness of the aluminum oxide film is reduced, the passivation structure with the excellent passivation performance is obtained, the conversion efficiency of the back contact battery based on the passivation structure is guaranteed, the EL fog black undesirable proportion is reduced, and the service life of the back contact battery is prolonged. And the use amount of silver paste is also reduced.

Description

Technical Field

[0001] The present invention relates to the field of back-contact batteries, and in particular to a surface passivation process for improving the conversion efficiency of back-contact batteries. Background Art

[0002] The back contact cell (Tunnel Back Contact Solar Cell) combines the tunnel oxide layer technology of TOPCon (Tunnel Oxide Passivated Contact) with the back contact electrode design of IBC (Interdigitated Back Contact). It not only retains the excellent passivation performance of TOPCon, but also inherits the characteristics of IBC's unobstructed high light receiving area. This combination significantly improves the cell's Voc (Open Circuit Voltage) and conversion efficiency. Although back contact cells have high conversion efficiency, their production cost is relatively high, and back contact cells face great pressure in market competition.

[0003] The current back-contact cell surface is passivated on both sides with aluminum oxide, and a silicon nitride film is applied to the aluminum oxide for protection and to provide passivation and anti-reflection properties. Generally speaking, thicker aluminum oxide can significantly enhance surface passivation performance and resist UVID (Ultraviolet Induced Degradation) risks. However, because the aluminum oxide deposited by ALD (Atomic Layer Deposition) is relatively dense, increasing thickness will affect the contact between the slurry and the doped polysilicon layer, requiring a higher amount of silver paste to ensure a lower contact resistance. This will increase the amount of silver paste used in the cell, resulting in a significant increase in non-silicon costs. In addition, thicker aluminum oxide will cause the cell EL (Electroluminescence) to appear foggy and black during the metallization process, seriously affecting the production line yield. Moreover, the front of the battery has a velvet structure, and the back is a boron-doped and phosphorus-doped polysilicon layer deposited on a polished surface. Due to differences in electrical polarity and morphology, the deposition of the aluminum oxide film will vary to a certain extent. Therefore, the deposition of the aluminum oxide film requires comprehensive consideration of multiple aspects to ensure comprehensive passivation of the battery surface, which undoubtedly increases the design difficulty of ALD.

[0004] Therefore, back-contact batteries have the problem of relatively high production costs, and their surface passivation process still needs to be optimized. Summary of the Invention

[0005] In response to the above problems, the present invention aims to provide a surface passivation process for improving the conversion efficiency of back-contact cells, thereby achieving the goal of reducing process costs while ensuring cell conversion efficiency. The technical solution is as follows:

[0006] On the one hand, a surface passivation process for improving the conversion efficiency of a back-contact cell is provided, comprising:

[0007] Step 1) aluminum oxide film deposition;

[0008] Step 2) silicon nitride film deposition;

[0009] In step 1), the aluminum oxide film deposition temperature is 200-260°C;

[0010] Specifically, 200-260°C is a relatively low aluminum oxide film deposition temperature. The embodiments of the present application take into account that the aluminum oxide deposition rate is faster under conditions of excessively high aluminum oxide film deposition temperatures, which is not conducive to the precise control of film thickness uniformity and may form non-layered growth. Under conditions of excessively high temperatures, the reaction is too intense, causing the precursor TMA (Trimethylaluminum) to react with other products before being completely decomposed into the required aluminum atoms and methane and other products, thereby generating carbon residues and affecting the purity of the aluminum oxide film; this avoids the premature escape of hydrogen atoms in the aluminum oxide film during deposition under high temperature conditions, reducing the available active hydrogen during subsequent annealing and weakening the chemical passivation effect; thereby reducing the thickness requirement of the aluminum oxide film and thereby reducing the amount of silver paste used.

[0011] In an exemplary embodiment, in step 1), the thickness of the aluminum oxide film is 4.7-5 nm.

[0012] In the embodiments of the present application, by regulating the deposition temperature of the aluminum oxide film, combined with the regulation of the thickness of the aluminum oxide film, its passivation performance is enhanced while being more conducive to metallization contact. A low-thickness, high-density aluminum oxide film is used to reduce the surface defect compounding of the passivation structure based on the silicon wafer, and the screen printing and sintering process forms good contact performance, thereby improving the photoelectric conversion efficiency of the back contact battery based on the passivation structure, reducing fog and black defects, and reducing the amount of silver paste used.

[0013] In one exemplary embodiment, in step 2), the silicon-nitrogen ratio in the top region of the front silicon nitride film is 1 / 9-1 / 16;

[0014] The silicon-nitrogen ratio in the bottom region of the front silicon nitride film is 1 / 4-1 / 7;

[0015] and / or,

[0016] The thickness of the top region of the front silicon nitride film is 41-55 nm;

[0017] The thickness of the bottom region of the front silicon nitride film is 15-25 nm.

[0018] In an exemplary embodiment, in step 2), the front silicon nitride film deposition temperature is 450-540°C.

[0019] In one exemplary embodiment, in step 2), the silicon-nitrogen ratio in the top layer region of the back silicon nitride film is 1 / 12-1 / 18;

[0020] The silicon-nitrogen ratio in the bottom region of the silicon nitride film is 1 / 2-1 / 5;

[0021] and / or,

[0022] The top region of the backside silicon nitride film has a thickness of 65-75 nm;

[0023] The thickness of the bottom region of the back silicon nitride film is 15-25 nm.

[0024] In an exemplary embodiment, in step 2), the back side silicon nitride film deposition temperature is 470-560°C.

[0025] On the other hand, the present invention also provides a passivation structure, including a passivation structure obtained by any of the above-mentioned surface passivation processes for improving the conversion efficiency of back-contact cells.

[0026] On the other hand, the present invention also provides a back-contact battery comprising the above-mentioned passivation structure.

[0027] The present invention discloses a surface passivation process for improving the conversion efficiency of back-contact solar cells, comprising: step 1) depositing an aluminum oxide film; and step 2) depositing a silicon nitride film. In step 1), the aluminum oxide film is deposited at a temperature of 200-260°C. By producing a highly dense aluminum oxide film at a relatively low deposition temperature, the present invention reduces the thickness requirement for the aluminum oxide film in back-contact solar cells, resulting in a passivation structure with excellent passivation performance. While maintaining the conversion efficiency of back-contact solar cells based on this passivation structure, the proportion of defective EL black fogging and the amount of silver paste used are reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] To more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present invention, and those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0029] Figure 1 This is a flow chart of a surface passivation process for improving the conversion efficiency of back-contact cells provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] The present invention provides a surface passivation process for improving the conversion efficiency of back contact cells. Figure 1 , Figure 1 The present invention is a flow chart of a surface passivation process for improving the conversion efficiency of back-contact cells, which is applied to the preparation of back-contact cells and includes the following steps:

[0032] Step 1) Aluminum oxide film deposition; wherein the aluminum oxide film deposition temperature is 200-260°C.

[0033] Most preferably, the aluminum oxide film deposition temperature is 200-260°C.

[0034] Specifically, an ultra-high-resistance N-type doped silicon wafer with concentrated resistivity can be used. After multiple cleaning, high-temperature and laser patterning processes, a pyramid velvet structure is formed on the front of the silicon wafer, and a finger-like structure is formed on the back of the silicon wafer, in which the boron-doped P region and the phosphorus-doped N region are isolated. The surface of the silicon wafer is then passivated. It should be noted that the front of the silicon wafer refers to the area of ​​the battery based on the silicon wafer that receives light, and the back of the silicon wafer refers to the non-light-receiving surface of the battery based on the silicon wafer (that is, the other side surface opposite to the front), which is the area where the battery realizes current collection, surface passivation and electrode connection. Regarding the area where the aluminum oxide film is deposited, it can include the front and back of the silicon wafer, and can also include the four sides of the silicon wafer. It can be set according to actual needs in specific implementation.

[0035] Specifically, the embodiment of the present application can use the ALD process to deposit aluminum oxide film. 200-260°C is a relatively low aluminum oxide film deposition temperature. The embodiment of the present application takes into account the use of a relatively low aluminum oxide film deposition temperature to avoid excessively high aluminum oxide film deposition temperature resulting in a faster aluminum oxide deposition rate, which is not conducive to the precise control of film thickness uniformity and may form non-layered growth; to avoid the precursor TMA from being incompletely decomposed, resulting in carbon residues, and affecting the purity of the film; to avoid the hydrogen atoms in aluminum oxide from escaping prematurely during aluminum oxide film deposition, reducing the active hydrogen available during subsequent annealing, and weakening the chemical passivation effect.

[0036] Step 2) Silicon nitride film deposition.

[0037] Specifically, the silicon nitride film includes a front silicon nitride film and a back silicon nitride film, wherein the front silicon nitride film refers to the silicon nitride film located on the front side of the silicon wafer, and the back silicon nitride film refers to the silicon nitride film located on the back side of the silicon wafer (for relevant descriptions on the front side and the back side of the silicon wafer, please refer to the above explanation).

[0038] In a specific implementation, the silicon-nitrogen ratio in the top region of the silicon nitride film is smaller than the silicon-nitrogen ratio in the bottom region of the silicon nitride film, where the top region of the silicon nitride film refers to a region away from the aluminum oxide film, and the bottom region of the silicon nitride film refers to a region in contact with the aluminum oxide film.

[0039] In the embodiment of the present application, in step 1), the thickness of the aluminum oxide film is 4.7-5 nm.

[0040] Specifically, a thicker aluminum oxide film can significantly enhance the surface passivation performance and resist the risk of UVID (ultraviolet induced degradation). Considering that the aluminum oxide film obtained by the ALD process in the embodiment of the present application is relatively dense, as the thickness of the aluminum oxide film increases, it will affect the contact between the slurry and the doped polysilicon layer, thereby requiring a higher amount of silver paste to ensure a smaller contact resistance, which will increase the amount of silver paste used in the battery (based on the EL brightness and defective type performance on the production line, it is usually necessary to increase the amount of silver paste to improve the contact effect and avoid EL fogging), resulting in a significant increase in non-silicon costs. In addition, a thicker aluminum oxide film will cause fogging defects in the battery EL during the metallization process, seriously affecting the production line yield. Therefore, the embodiment of the present application controls the deposition temperature of the aluminum oxide film while controlling the thickness of the aluminum oxide film. The embodiment of the present application finds that when the thickness of the aluminum oxide film is lower than 4.7 nm, there is a risk of UVID, and when the thickness of the aluminum oxide film is higher than 5 nm, the contact performance of the slurry will deteriorate. The thickness of the aluminum oxide film obtained by deposition at an aluminum oxide film deposition temperature of 200-260°C is 4.7-5 nm, which enhances its passivation performance and is more conducive to metallization contact. A low-thickness and high-density aluminum oxide film is used to reduce the defect compounding on the surface of the battery cell, and form good contact performance during the screen printing and sintering process, thereby improving the photoelectric conversion efficiency of the battery, reducing fog and black defects, and reducing the amount of silver paste used.

[0041] In an embodiment of the present application, in step 2), the silicon-nitrogen ratio in the top layer region of the front silicon nitride film is 1 / 9-1 / 16; among which, 1 / 10-1 / 15 is preferred, and 1 / 12 is most preferred.

[0042] The silicon-nitrogen ratio in the bottom region of the front silicon nitride film is 1 / 4-1 / 7; among them, 1 / 4-1 / 6 is preferred, and 1 / 4 is most preferred.

[0043] and / or,

[0044] The thickness of the top region of the front silicon nitride film is 41-55 nm.

[0045] The thickness of the bottom region of the front silicon nitride film is 15-25 nm.

[0046] In a specific implementation, the silicon-nitrogen ratio can gradually increase from the top region of the silicon nitride film to the bottom region of the silicon nitride film. It should be noted that the silicon nitride film can include a top region and a bottom region of the silicon nitride film, or can include multiple regions. For example, at least one region can be added between the top region and the bottom region of the silicon nitride film to provide a gradient distribution of the silicon-nitrogen ratio in the silicon nitride film.

[0047] The front side of the cell's silicon wafer has a simple, velvet-like structure. Compared to the polished back side, it has more velvet defects and dangling bonds, requiring more hydrogen atoms to combine with the dangling bonds and passivate interface defects. Therefore, the front side silicon nitride film uses a high silicon-nitrogen ratio to increase its refractive index, thereby increasing the hydrogen content in the film and enhancing the passivation effect. The silicon-nitrogen ratio in the front side silicon nitride film is designed to be gradient (the silicon-nitrogen ratio in the top region of the front side silicon nitride film is lower than that in the bottom region of the front side silicon nitride film), thereby reducing the solar reflectivity of the cell surface.

[0048] In the embodiment of the present application, in step 2), the front silicon nitride film deposition temperature is 450-540°C, preferably 470-490°C.

[0049] The PECVD (Plasma-Enhanced Chemical Vapor Deposition) process for front-side silicon nitride film deposition uses a relatively low deposition temperature to prepare silicon nitride films, which can reduce the escape of hydrogen atoms in the aluminum oxide film, achieve complementary passivation with the hydrogen in the aluminum oxide film, and reduce interface defects.

[0050] In an embodiment of the present application, in step 2), the silicon-nitrogen ratio in the top layer region of the back silicon nitride film is 1 / 12-1 / 18; among them, preferably 1 / 13-1 / 17, and most preferably 1 / 16.

[0051] The silicon-nitrogen ratio in the bottom region of the silicon nitride film is 1 / 2-1 / 5, preferably 1 / 3-1 / 4, and most preferably 1 / 3.

[0052] and / or,

[0053] The top region of the backside silicon nitride film has a thickness of 65-75 nm.

[0054] The thickness of the bottom region of the back silicon nitride film is 15-25 nm.

[0055] The back structure of the silicon wafer of the battery is complex. The cross-finger doped polysilicon forms P+ / N+ regions with the silicon base respectively. The aluminum oxide film is used for field passivation and chemical passivation respectively. The silicon nitride film carries a positive charge and can provide field passivation for the N+ region.

[0056] The bottom area of ​​the back silicon nitride film adopts a high silicon-nitrogen ratio to increase its refractive index, which enhances the passivation effect while also allowing more long-wave light that is about to be refracted out of the battery to be reflected back into the battery, increasing the battery's light absorption and thus improving the battery conversion efficiency; the top area of ​​the back silicon nitride film adopts a low silicon-nitrogen ratio to avoid direct contact between the high-refractive layer and the slurry, which may cause erosion and damage to the film layer and reduce the passivation performance.

[0057] Taking the following embodiment 1 as an example compared with the comparative example, the silicon-nitrogen ratio in the bottom area of ​​the back silicon nitride film is increased, the refractive index of the silicon nitride film is high, and the H passivation effect is better. At the same time, the high-refractive-index silicon nitride film has better conductivity, and the silicon content is relatively high, which is conducive to the silver paste penetrating the film to reduce contact resistance and improve contact performance. At the same time, the overall thickness of the film layer is slightly reduced, which is conducive to the improvement of metallization contact performance.

[0058] In the embodiment of the present application, in step 2), the back side silicon nitride film deposition temperature is 470-560°C.

[0059] Because the back structure of the silicon wafer used to prepare the battery is complex, the cross-finger doped polysilicon forms P+ / N+ regions with the silicon base respectively, and the aluminum oxide film is field passivated and chemically passivated thereon respectively. The silicon nitride film can provide field passivation for the N+ region because it carries a positive charge. In the embodiment of the present application, on the silicon wafer, the aluminum oxide film deposited on the back is thicker than the aluminum oxide film deposited on the front (about 1.7 times that of the front), and the aluminum oxide film deposited on the back also contains relatively more H atoms. The PECVD process for depositing the silicon nitride film on the back uses a relatively high deposition temperature to prepare the silicon nitride film, allowing the hydrogen atoms in the aluminum oxide film to escape before the silicon nitride film is deposited, avoiding defects caused by excessive escape during high-temperature sintering. It should be noted here that the aluminum oxide film deposited on the back is located on the back of the silicon wafer, and the aluminum oxide film deposited on the front is located on the front of the silicon wafer.

[0060] On the other hand, the present invention further provides a passivation structure, including a passivation structure obtained by any of the above-mentioned surface passivation processes.

[0061] Specifically, the passivation structure includes a silicon wafer and an aluminum oxide film and a silicon nitride film formed on the silicon wafer by any of the above-mentioned surface passivation processes.

[0062] The present invention has no special requirements for the preparation methods of other parts of the passivation structure provided in the embodiments of the present application except for the surface passivation process of the aluminum oxide film and the silicon nitride film formed on the silicon wafer. The conventional silicon wafer preparation method in the field can be adopted. The specific preparation process is well known in the silicon wafer field and will not be described in detail here.

[0063] On the other hand, the present invention also provides a back-contact battery comprising the above-mentioned passivation structure.

[0064] The present invention has no special requirements for the preparation method of the back-contact battery, and the conventional preparation method of the back-contact battery in the art can be adopted. The specific preparation process is well known in the battery field and will not be described here in detail.

[0065] The technical solution of the present application is described below with reference to specific embodiments and comparative examples.

[0066] The following embodiments are used to illustrate a surface passivation process, a passivation structure, and a back-contact cell for improving the conversion efficiency of a back-contact cell according to the present invention.

[0067] Example 1

[0068] After the silicon wafer (used in back-contact cells) is isolated from the P and N regions on the back and a textured structure is formed on the front, it is placed in the ALD system chamber using automated equipment. TMA and H2O are alternately introduced using nitrogen to deposit a thin layer of aluminum oxide film on both the front and back of the silicon wafer. The pulse time of TMA is 5s and the purge time is 12s, the pulse time of H2O is 5s and the purge time is 12s, the aluminum oxide film deposition temperature is 250°C, the number of cycles is set to 38, and the thickness of the aluminum oxide film is 4.75nm.

[0069] After the aluminum oxide film deposition is completed, the silicon wafer with the aluminum oxide film is placed in the PECVD system for deposition of silicon nitride films on the front and back sides.

[0070] The deposition temperature of the front silicon nitride film PECVD process is set to 470-490°C (it should be noted here that the furnace tube used for silicon nitride film deposition is relatively long, and the temperature setting is composed of multiple temperature zones, gradually decreasing from the furnace mouth to the furnace tail). The front silicon nitride film is set to a two-layer structure, and the silicon-nitrogen ratio in the bottom area of ​​the front silicon nitride film is set to 1 / 4, and the thickness of the bottom area of ​​the front silicon nitride film is 20nm; the silicon-nitrogen ratio in the top area of ​​the front silicon nitride film is set to 1 / 12, and the thickness of the top area of ​​the front silicon nitride film is 46nm. The overall thickness of the front silicon nitride film is 66nm, and the refractive index is 2.15.

[0071] The deposition temperature of the back PECVD is set to 490-530°C, the back silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the back silicon nitride film is set to 1 / 3, and the thickness of the bottom area of ​​the back silicon nitride film is 21nm; the silicon-nitrogen ratio in the top area of ​​the back silicon nitride film is set to 1 / 16, and the thickness of the top area of ​​the back silicon nitride film is 69nm. The overall thickness of the back silicon nitride film is 90nm, and the refractive index is 2.03.

[0072] After the deposition of the front and back silicon nitride films is completed, subsequent processes such as metallization, testing and sorting are carried out to complete the preparation of the back contact battery.

[0073] Example 2

[0074] Compared with Example 1, the aluminum oxide film deposition temperature is 160° C., and other conditions remain unchanged.

[0075] Example 3

[0076] Compared with Example 1, the aluminum oxide film deposition temperature is 300° C., and other conditions remain unchanged.

[0077] Example 4

[0078] Compared with Example 1, the aluminum oxide film deposition temperature is 200° C., and other conditions remain unchanged.

[0079] Example 5

[0080] Compared with Example 1, the aluminum oxide film deposition temperature is 260° C., and other conditions remain unchanged.

[0081] Example 6

[0082] Compared with Example 1, the aluminum oxide film deposition temperature is 240° C., and other conditions remain unchanged.

[0083] Example 7

[0084] Compared with Example 1, the number of cycles was set to 30, the thickness of the aluminum oxide film was set to 1.63 nm, and other conditions remained unchanged.

[0085] Example 8

[0086] Compared with Example 1, the number of cycles was set to 50, the thickness of the aluminum oxide film was set to 6.25 nm, and other conditions remained unchanged.

[0087] Example 9

[0088] Compared with Example 1, the number of cycles was set to 42, the thickness of the aluminum oxide film was set to 5.25 nm, and other conditions remained unchanged.

[0089] Example 10

[0090] Compared with Example 1, the deposition temperature of the front silicon nitride film PECVD process is set to 450-540° C., the refractive index is 2.15, and other conditions remain unchanged.

[0091] Example 11

[0092] Compared with Example 1, the silicon-nitrogen ratio in the bottom region of the front silicon nitride film is set to 2 / 7, the refractive index is 2.37, and other conditions remain unchanged.

[0093] Example 12

[0094] Compared with Example 1, the bottom layer area of ​​the front silicon nitride film is set to 1 / 6, the refractive index is 2.18, and other conditions remain unchanged.

[0095] Example 13

[0096] Compared with Example 1, the bottom layer area of ​​the front silicon nitride film is set to 1 / 7, the refractive index is 2.15, and other conditions remain unchanged.

[0097] Example 14

[0098] Compared with Example 1, the thickness of the bottom region of the front silicon nitride film is 15 nm, the refractive index is 2.25, and other conditions remain unchanged.

[0099] Example 15

[0100] Compared with Example 1, the thickness of the bottom region of the front silicon nitride film is 25 nm, and the refractive index is 2.25, while other conditions remain unchanged.

[0101] Example 16

[0102] Compared with Example 1, the thickness of the bottom region of the front silicon nitride film is 18 nm, and the refractive index is 2.25, while other conditions remain unchanged.

[0103] Example 17

[0104] Compared with Example 1, the thickness of the bottom region of the front silicon nitride film is 22 nm, and the refractive index is 2.25, while other conditions remain unchanged.

[0105] Example 18

[0106] Compared with Example 1, the silicon-nitrogen ratio in the top region of the front silicon nitride film is set to 1 / 9, the refractive index is 2.11, and other conditions remain unchanged.

[0107] Example 19

[0108] Compared with Example 1, the silicon-nitrogen ratio in the top region of the front silicon nitride film is set to 1 / 16, the refractive index is 1.93, and other conditions remain unchanged.

[0109] Example 20

[0110] Compared with Example 1, the silicon-nitrogen ratio in the top region of the front silicon nitride film is set to 1 / 10, the refractive index is 2.07, and other conditions remain unchanged.

[0111] Example 21

[0112] Compared with Example 1, the silicon-nitrogen ratio in the top region of the front silicon nitride film is set to 1 / 15, the refractive index is 1.99, and other conditions remain unchanged.

[0113] Example 22

[0114] Compared with Example 1, the thickness of the top region of the front silicon nitride film is 41 nm, the refractive index is 2.02, and other conditions remain unchanged.

[0115] Example 23

[0116] Compared with Example 1, the thickness of the top region of the front silicon nitride film is 55 nm, and the refractive index is 2.02, while other conditions remain unchanged.

[0117] Example 24

[0118] Compared with Example 1, the thickness of the top region of the front silicon nitride film is 44 nm, and the refractive index is 2.02, while other conditions remain unchanged.

[0119] Example 25

[0120] Compared with Example 1, the thickness of the top region of the front silicon nitride film is 52 nm, and the refractive index is 2.02, while other conditions remain unchanged.

[0121] Example 26

[0122] Compared with Example 1, the deposition temperature of the back PECVD was set to 470-560° C., the refractive index was 2.03, and other conditions remained unchanged.

[0123] Example 27

[0124] Compared with Example 1, the silicon-nitrogen ratio in the bottom region of the back silicon nitride film is set to 1 / 2, the refractive index is 2.5, and other conditions remain unchanged.

[0125] Example 28

[0126] Compared with Example 1, the silicon-nitrogen ratio in the bottom region of the back silicon nitride film is set to 1 / 4, the refractive index is 2.25, and other conditions remain unchanged.

[0127] Example 29

[0128] Compared with Example 1, the silicon-nitrogen ratio in the bottom region of the back silicon nitride film is set to 1 / 5, the refractive index is 2.20, and other conditions remain unchanged.

[0129] Example 30

[0130] Compared with Example 1, the thickness of the bottom region of the back silicon nitride film is 15 nm, the refractive index is 2.45, and other conditions remain unchanged.

[0131] Example 31

[0132] Compared with Example 1, the thickness of the bottom region of the back silicon nitride film is 25 nm, and the refractive index is 2.45, while other conditions remain unchanged.

[0133] Example 32

[0134] Compared with Example 1, the thickness of the bottom region of the back silicon nitride film is 18 nm, and the refractive index is 2.45, while other conditions remain unchanged.

[0135] Example 33

[0136] Compared with Example 1, the thickness of the bottom region of the back silicon nitride film is 22 nm, and the refractive index is 2.45, while other conditions remain unchanged.

[0137] Example 34

[0138] Compared with Example 1, the silicon-nitrogen ratio in the top region of the back silicon nitride film is set to 1 / 12, the refractive index is 2.02, and other conditions remain unchanged.

[0139] Example 35

[0140] Compared with Example 1, the silicon-nitrogen ratio in the top region of the back silicon nitride film is set to 1 / 18, the refractive index is 1.87, and other conditions remain unchanged.

[0141] Example 36

[0142] Compared with Example 1, the silicon-nitrogen ratio in the top region of the back silicon nitride film is set to 1 / 13, the refractive index is 1.98, and other conditions remain unchanged.

[0143] Example 37

[0144] Compared with Example 1, the silicon-nitrogen ratio in the top region of the back silicon nitride film is set to 1 / 17, the refractive index is 1.90, and other conditions remain unchanged.

[0145] Example 38

[0146] Compared with Example 1, the thickness of the top region of the back silicon nitride film is 65 nm, the refractive index is 1.93, and other conditions remain unchanged.

[0147] Example 39

[0148] Compared with Example 1, the thickness of the top region of the back silicon nitride film is 75 nm, and the refractive index is 1.93, while other conditions remain unchanged.

[0149] Example 40

[0150] Compared with Example 1, the thickness of the top region of the back silicon nitride film is 67 nm, and the refractive index is 1.93, while other conditions remain unchanged.

[0151] Example 41

[0152] Compared with Example 1, the thickness of the top region of the back silicon nitride film is 72 nm, and the refractive index is 1.93, while other conditions remain unchanged.

[0153] Comparative Example 1

[0154] After the silicon wafer completes the isolation of the P and N regions on the back and the formation of a velvet structure on the front, it is placed in the ALD system chamber by automated equipment. TMA and H2O are alternately introduced by nitrogen to deposit a thin layer of aluminum oxide film on both the front and back of the battery. The pulse time of TMA is 5s and the purge time is 12s. The pulse time of H2O is 5s and the purge time is 12s. The aluminum oxide film deposition temperature is set to 210°C, the number of cycles is set to 45, and the thickness of the aluminum oxide film is 5.625nm.

[0155] After the aluminum oxide film deposition is completed, the silicon wafer is placed in the PECVD system for deposition of silicon nitride films on the front and back sides.

[0156] The deposition temperature of the PECVD process of the front silicon nitride film is set to 530-550°C, the front silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the front silicon nitride film is set to 1 / 6, and the thickness of the bottom area of ​​the front silicon nitride film is 18nm; the silicon-nitrogen ratio in the top area of ​​the front silicon nitride film is set to 1 / 16, the thickness of the top area of ​​the front silicon nitride film is 50nm, the overall thickness of the front silicon nitride film is 68nm, and the refractive index is 2.13.

[0157] The deposition temperature of the PECVD process for the back side silicon nitride film is set to 460-500°C, the back side silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom region of the back side silicon nitride film is set to 1 / 5, and the thickness of the bottom region of the back side silicon nitride film is 20nm; the silicon-nitrogen ratio in the top region of the back side silicon nitride film is set to 1 / 18, and the thickness of the top region of the back side silicon nitride film is 75nm. The overall thickness of the back side silicon nitride film is 95nm, and the refractive index is 1.98.

[0158] After the deposition of silicon nitride on the front and back sides is completed, a passivation structure is obtained. The passivation structure is subjected to subsequent processes such as metallization, testing and sorting to complete the preparation of the back contact battery.

[0159] Comparative Example 2

[0160] After the silicon wafer completes the isolation of the P and N regions on the back and the formation of a velvet structure on the front, it is placed in the ALD system chamber by automated equipment. TMA and H2O are alternately introduced by nitrogen to deposit a thin layer of aluminum oxide film on both the front and back of the battery. The pulse time of TMA is 5s and the purge time is 12s. The pulse time of H2O is 5s and the purge time is 12s. The aluminum oxide film deposition temperature is set to 250°C, the number of cycles is set to 38, and the thickness of the aluminum oxide film is 4.75nm.

[0161] After the aluminum oxide film deposition is completed, the silicon wafer is placed in the PECVD system to deposit silicon nitride film on the front and back sides.

[0162] The deposition temperature of the PECVD process of the front silicon nitride film is set to 530-550°C, the front silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the front silicon nitride film is set to 1 / 6, and the thickness of the bottom area of ​​the front silicon nitride film is 18nm; the silicon-nitrogen ratio in the top area of ​​the front silicon nitride film is set to 1 / 16, and the thickness of the top area of ​​the front silicon nitride film is 50nm. The overall front silicon nitride thickness is 68nm, and the refractive index is 2.13.

[0163] The deposition temperature of the PECVD process of the back side silicon nitride film is set to 460-500°C, the back side silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the back side silicon nitride film is set to 1 / 5, and the thickness of the silicon nitride film in the bottom area of ​​the back side silicon nitride film is 20nm; the top area of ​​the back side silicon nitride film is set to 1 / 18, and the thickness of the top area of ​​the back side silicon nitride film is 75nm. The overall thickness of the back side silicon nitride film is 95nm, and the refractive index is 1.98.

[0164] After the deposition of silicon nitride on the front and back sides is completed, a passivation structure is obtained. The passivation structure is subjected to subsequent processes such as metallization, testing and sorting to complete the preparation of the back contact battery.

[0165] Comparative Example 3

[0166] After the silicon wafer completes the isolation of the P and N regions on the back and the formation of a velvet structure on the front, it is placed in the ALD system chamber by automated equipment. TMA and H2O are alternately introduced by nitrogen to deposit a thin layer of aluminum oxide film on both the front and back of the battery. The pulse time of TMA is 5s and the purge time is 12s. The pulse time of H2O is 5s and the purge time is 12s. The aluminum oxide film deposition temperature is set to 210°C, the number of cycles is set to 45, and the thickness of the aluminum oxide film is 5.625nm.

[0167] After the aluminum oxide film deposition is completed, the silicon wafer is placed in the PECVD system to deposit silicon nitride film on the front and back sides.

[0168] The deposition temperature of the PECVD process of the front silicon nitride film is set to 470-490°C, the front silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the front silicon nitride film is set to 1 / 4, and the thickness of the bottom area of ​​the front silicon nitride film is 20nm; the silicon-nitrogen ratio in the top area of ​​the front silicon nitride film is set to 1 / 12, the thickness of the top area of ​​the front silicon nitride film is 46nm, the overall front silicon nitride thickness is 66nm, and the refractive index is 2.15.

[0169] The deposition temperature of the PECVD process for the back side silicon nitride film is set to 460-500°C, the back side silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom region of the back side silicon nitride film is set to 1 / 5, and the thickness of the bottom region of the back side silicon nitride film is 20nm; the silicon-nitrogen ratio in the top region of the back side silicon nitride film is set to 1 / 18, and the thickness of the top region of the back side silicon nitride film is 75nm. The overall thickness of the back side silicon nitride film is 95nm, and the refractive index is 1.98.

[0170] After the deposition of silicon nitride on the front and back sides is completed, a passivation structure is obtained. The passivation structure is subjected to subsequent processes such as metallization, testing and sorting to complete the preparation of the back contact battery.

[0171] Comparative Example 4

[0172] After the silicon wafer completes the isolation of the P and N regions on the back and the formation of a velvet structure on the front, it is placed in the ALD system chamber by automated equipment. TMA and H2O are alternately introduced by nitrogen to deposit a thin layer of aluminum oxide film on both the front and back of the battery. The pulse time of TMA is 5s and the purge time is 12s. The pulse time of H2O is 5s and the purge time is 12s. The aluminum oxide film deposition temperature is set to 210°C, the number of cycles is set to 45, and the thickness of aluminum oxide is 5.625nm.

[0173] After the aluminum oxide film deposition is completed, the silicon wafer is placed in the PECVD system to deposit silicon nitride film on the front and back sides.

[0174] The deposition temperature of the front PECVD is set to 530-550°C, the front silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the front silicon nitride film is set to 1 / 6, and the thickness of the bottom area of ​​the front silicon nitride film is 18nm; the silicon-nitrogen ratio in the top area of ​​the front silicon nitride film is set to 1 / 16, and the thickness of the top area of ​​the front silicon nitride film is 50nm. The overall thickness of the front silicon nitride film is 68nm, and the refractive index is 2.13.

[0175] The deposition temperature of the back PECVD is set to 490-530°C, the back silicon nitride film is set to a two-layer structure, the silicon-nitrogen ratio in the bottom area of ​​the back silicon nitride film is set to 1 / 3, and the thickness of the bottom area of ​​the back silicon nitride film is 21nm; the silicon-nitrogen ratio in the top area of ​​the back silicon nitride film is set to 1 / 16, and the thickness of the top area of ​​the back silicon nitride film is 69nm. The overall thickness of the back silicon nitride film is 90nm, and the refractive index is 2.03.

[0176] After the deposition of the front and back silicon nitride films is completed, a passivation structure is obtained. The passivation structure is subjected to subsequent processes such as metallization, testing and sorting to complete the preparation of the back contact battery.

[0177] Comparative Example 5

[0178] Compared with Example 1, the aluminum oxide film deposition temperature is 150° C., and other conditions remain unchanged.

[0179] Comparative Example 6

[0180] Compared with Example 1, the aluminum oxide film deposition temperature is 310° C., and other conditions remain unchanged.

[0181] The above-mentioned embodiments and comparative examples use the same batch of N-type 182*105 silicon wafers. Example 1 is the preferred embodiment of the present invention. Unless otherwise specified, the process schemes and parameters used in other process steps of the embodiments and comparative examples are the same. The performance test results of the back-contact cells obtained based on different embodiments and comparative examples are shown in Table 1. From the comparison results, the surface passivation process using the preferred embodiment of the present invention can significantly improve the photoelectric conversion efficiency of the back-contact cells, and the EL fog black defect is reduced by more than 90%. At the same time, the amount of silver paste used can be reduced by about 2 mg / piece. From a cost perspective, the lower the amount of slurry used, the better. However, if it is too low, poor contact will occur, resulting in EL fog black and low efficiency. The range of silver paste dosage is usually determined based on a lower level of EL fog black.

[0182] The present invention mainly reduces the amount of silver paste without affecting the contact effect by improving the ALD process, thereby achieving the purpose of cost reduction.

[0183] The bottom region of the silicon nitride film uses a higher silicon-nitrogen ratio, increasing its refractive index and enhancing the H passivation effect. The bottom region of the silicon nitride film is also relatively dense. The top region of the silicon nitride film uses a lower silicon-nitrogen ratio, making it relatively loose, reducing parasitic absorption in the top region of the silicon nitride film. Combined with this double-layer structure, the reflectivity of the solar cell surface is reduced. Compared to the comparative example, the silicon-nitrogen ratio in the embodiment is increased, resulting in a higher refractive index of the silicon nitride film and better H passivation. To prevent increased parasitic absorption in the film layer, the overall thickness of the silicon nitride film is slightly reduced, allowing more sunlight to enter the solar cell, improving sunlight utilization and boosting the cell's photoelectric conversion efficiency.

[0184] Table 1. Performance test results of back contact cells obtained based on different embodiments and comparative examples

[0185]

[0186]

[0187]

[0188] In Comparative Example 1, the photoelectric conversion efficiency of the back-contact battery is 27.49%. In Comparative Example 2, the ALD process adopts the preferred embodiment of the present invention, aiming to reduce the thickness of the aluminum oxide film and thus improve the contact performance between the silver paste and the doped area in the metallization process. On the one hand, it can reduce the amount of silver paste used in the production line, and on the other hand, it can reduce the proportion of EL fog and black defects in the production line. However, simply reducing the thickness of the aluminum oxide film will lead to a deterioration of the passivation effect, thereby causing a decrease in the open-circuit voltage of the battery. Therefore, the present invention increases the deposition temperature of the aluminum oxide film in the ALD process, improves the density of the prepared aluminum oxide film, and reduces the defect recombination centers on the surface of the silicon wafer. While reducing the thickness of the aluminum oxide film, its passivation performance not only does not deteriorate but is improved. As shown in Table 1, compared with the performance data of Comparative Example 1, the open circuit voltage of the back contact battery in Comparative Example 2 is increased by 1mV, the filling is increased by 0.02%, the amount of silver paste used for each back contact battery in the metallization process is reduced by 2mg, and the EL fog black defect ratio is reduced by more than 50%, indicating that the improvement of the ALD process improves the metallization contact performance while also improving its passivation ability on the back contact battery surface.

[0189] During the ALD process, aluminum oxide films are deposited on both the front and back surfaces of the battery. The process of the front and back silicon nitride films must take into account their impact on the aluminum oxide films. Therefore, the deposition processes of the front and back silicon nitride films need to be optimized and designed separately to give full play to their passivation and anti-reflection properties. In Comparative Example 3, on the one hand, increasing the silicon-nitrogen ratio of the silicon nitride film increases its refractive index, thereby increasing the H passivation effect; on the other hand, lowering the furnace tube temperature (i.e., the aluminum oxide film deposition temperature). The lower furnace tube temperature can reduce the H escape in the aluminum oxide film, allowing it to reach more battery interface defects during the sintering process, reducing surface defect recombination centers, and the deposition rate of the silicon nitride film is slower at a lower temperature, which is beneficial to the control of the uniformity of the silicon nitride film; furthermore, slightly reducing the thickness of the silicon nitride film can reduce the parasitic absorption of sunlight by the film layer, allowing more sunlight to enter the battery interior to improve utilization. Compared with Comparative Example 1, the conversion efficiency of the battery produced in Comparative Example 3 is increased by 0.07%, mainly manifested in a 0.9mV increase in Voc, indicating that the high-refractive-index silicon nitride film deposited at a lower temperature improves the excellent H passivation performance of the back-contact battery surface.

[0190] In Comparative Example 4, the back-side silicon nitride film utilizes a higher furnace temperature to prematurely release metastable hydrogen from the back-side aluminum oxide, preventing it from escaping from the film during the sintering process and causing poor EL performance. This also increases the silicon-to-nitrogen ratio of the back-side silicon nitride film, raising its refractive index and enhancing the film's hydrogen passivation effect on the back side of the cell. Furthermore, reducing the thickness of the silicon nitride film facilitates the formation of an ohmic contact during the metallization process, thereby reducing contact resistance. Compared to Comparative Example 1, the cell fabricated in Comparative Example 4 exhibits a 0.5 mV increase in Voc, a 0.03% increase in FF, and a 40% reduction in the proportion of poor EL performance. This demonstrates that this back-side silicon nitride process improves the surface passivation of back-contact cells while also reducing the proportion of poor EL performance.

[0191] The embodiment of the present invention combines the process scheme of comparative examples 2 / 3 / 4 to increase the density and passivation performance of the aluminum oxide film in the ALD process, increase the hydrogen passivation of the front and back silicon nitride films, control the hydrogen escape in the aluminum oxide film, reduce the thickness of the aluminum oxide and silicon nitride film layers, and facilitate the contact performance of the metallization process. At the same time, the parasitic light absorption of the film layer is reduced. The photoelectric conversion efficiency of the battery can reach 27.60%, which is 0.11% higher than that of the prior art. Among them, the open circuit voltage is increased by 1.5mV, the short circuit current is increased by 4mA, the fill factor is increased by 0.12%, the battery silver paste consumption is reduced by 2mg / piece, and the EL fog black defect ratio is reduced by more than 90%, which is conducive to the industrialization and promotion of back contact batteries.

[0192] In summary, the technical solution of the present invention can significantly improve the photoelectric conversion efficiency and yield of back-contact batteries, improve product quality and customer satisfaction, and has high promotion value and economic benefits.

[0193] As can be seen from the above technical solutions of the embodiments of the present invention, the present invention discloses a surface passivation process for improving the conversion efficiency of back-contact solar cells, comprising: step 1) depositing an aluminum oxide film; and step 2) depositing a silicon nitride film. In step 1), the aluminum oxide film is deposited at a temperature of 200-260°C. By producing a highly dense aluminum oxide film at a relatively low aluminum oxide film deposition temperature, the present invention reduces the thickness requirement of the aluminum oxide film for back-contact solar cells, resulting in a passivation structure with excellent passivation performance. While maintaining the conversion efficiency of back-contact solar cells based on this passivation structure, it also reduces the proportion of defective EL fog and black, and reduces the amount of silver paste used.

[0194] The above description fully discloses the specific embodiments of the present invention. It should be noted that any modifications made by those skilled in the art to the specific embodiments of the present invention do not depart from the scope of the claims. Accordingly, the scope of the claims of the present invention is not limited solely to the foregoing specific embodiments.

Claims

1. A surface passivation process for improving the conversion efficiency of back-contact solar cells, comprising the following steps: Step 1) aluminum oxide film deposition; Step 2) silicon nitride film deposition; In step 1), the aluminum oxide film is deposited at a temperature of 200-260°C.

2. The surface passivation process for improving the conversion efficiency of back-contact cells according to claim 1, wherein in step 1), the thickness of the aluminum oxide film is 4.7-5 nm.

3. The surface passivation process for improving the conversion efficiency of back-contact solar cells according to claim 1, wherein in step 2), the silicon-nitrogen ratio in the top region of the front silicon nitride film is 1 / 9-1 / 16; The silicon-nitrogen ratio in the bottom region of the front silicon nitride film is 1 / 4-1 / 7; and / or, The thickness of the top region of the front silicon nitride film is 41-55 nm; The thickness of the bottom region of the front silicon nitride film is 15-25 nm.

4. The surface passivation process for improving the conversion efficiency of back-contact solar cells according to claim 3, wherein in step 2), the front silicon nitride film deposition temperature is 450-540°C.

5. The surface passivation process for improving the conversion efficiency of back-contact solar cells according to claim 1, wherein in step 2), the silicon-nitrogen ratio in the top region of the back silicon nitride film is 1 / 12-1 / 18; The silicon-nitrogen ratio in the bottom region of the silicon nitride film is 1 / 2-1 / 5; and / or, The thickness of the top region of the back silicon nitride film is 65-75 nm; The thickness of the bottom region of the back silicon nitride film is 15-25 nm.

6. The surface passivation process for improving the conversion efficiency of back-contact cells according to claim 5, wherein in step 2), the back silicon nitride film deposition temperature is 470-560°C.

7. A passivation structure, characterized in that: The passivation structure comprises a surface passivation process for improving the conversion efficiency of a back contact cell as described in any one of claims 1 to 6.

8. A back contact battery, characterized in that: A passivation structure comprising the one described in claim 7.

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