Heterojunction battery structure and anti-attenuation method

Abstract

The invention discloses a heterojunction cell structure and an anti-attenuation method, and relates to the field of solar cells. SiOx-TCO-SiNx gradient refraction thin film layers are arranged on doped amorphous / microcrystalline thin film layers of a backlight surface and a light receiving surface of a monocrystalline silicon wafer substrate; the SiOx-TCO-SiNx gradient refraction thin film layer sequentially comprises a SiNx thin film, a TCO thin film and a SiOx thin film from the light receiving surface to the inside; and the refractive indexes of the SiNx thin film, the TCO thin film and the SiOx thin film are sequentially reduced in a gradient manner. The conversion efficiency of the heterojunction cell is improved, and the weather resistance of the heterojunction cell assembly is enhanced. By adding a layer of composite gradient refraction film, the absorption of light is increased, water vapor is isolated, and the influence of the water vapor on the TCO film layer is reduced, so that the reliability of the HJT battery is improved.

Description

Technical Field

[0001] This invention relates to the field of solar cells, and more specifically to a heterojunction cell structure and an anti-degradation method. Background Technology

[0002] With the development of solar cell technology, the development of high-efficiency cells is receiving increasing attention. PERC cells, after years of development, have reached near-limit conversion efficiency. Meanwhile, N-type technology is gradually penetrating the market, steadily increasing its market share. Topcon N-type cells, with their excellent production line compatibility, have seen rapid capacity expansion and a market share exceeding 70%. Heterojunction (HJT) cells, with their high efficiency, high bifaciality, low degradation, and low temperature coefficient, are widely considered a strong contender for the next generation of mainstream solar cell technology. With the rapid advancement and maturation of cost-reduction technologies such as silver-clad copper, thinning, and domestic equipment manufacturing, the cost of HJT is quickly approaching that of PERC and Topcon. More importantly, HJT, as the ideal carrier for perovskite tandem cells, opens the door to ultra-high efficiencies exceeding 30%, representing the future direction of photovoltaic technology.

[0003] Heterojunction solar cells, also known as crystalline silicon heterojunction solar cells, are a type of hybrid technology that combines two different semiconductor materials to form a high-efficiency solar cell. Crystalline silicon (c-Si) serves as the substrate, typically a mature N-type monocrystalline silicon wafer, responsible for absorbing most of the sunlight and generating current. Amorphous silicon (a-Si) is deposited as a thin film on both sides of the crystalline silicon substrate, performing a crucial function—passivation. The "heterojunction" in heterojunction solar cells refers to the fact that crystalline silicon and amorphous silicon are two materials with different forms and band structures. The interface formed by their combination is called a heterojunction. The most distinctive feature of heterojunction solar cells is their symmetrical bifacial structure. The basic structure (from bottom to top) consists of a transparent conductive oxide film, a P-type amorphous silicon film (or an N-type amorphous silicon film), an intrinsic amorphous silicon film, and an N-type crystalline silicon substrate on the back side; and an N-type crystalline silicon substrate, an intrinsic amorphous silicon film, an N-type amorphous silicon film, a transparent conductive oxide film, and fine grid electrodes (usually using low-temperature silver paste) on the front side (the light-receiving side).

[0004] Traditional heterojunction solar cells have a transparent conductive film—TCO film—on their surface. The TCO film has weak adhesion to traditional encapsulation films and is sensitive to moisture, resulting in poor module reliability and a tendency to develop white spots, delamination, and other issues, leading to poor weather resistance. Summary of the Invention

[0005] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a heterojunction battery structure and an anti-degradation method, solving at least one technical problem mentioned in the background art.

[0006] (II) Technical Solution The technical solution adopted in this invention provides a heterojunction battery structure and an anti-degradation method: A first aspect provides a heterojunction battery structure, the battery structure including SiOx-TCO-SiNx gradient refractive thin film layers disposed on both the backlight surface and the light-receiving surface of a monocrystalline silicon wafer substrate. The SiOx-TCO-SiNx gradient refractive thin film layer comprises, inwardly from the light-receiving surface, a SiNx thin film, a TCO thin film, and a SiOx thin film; Furthermore, the refractive indices of the SiNx thin film, TCO thin film, and SiOx thin film decrease in that order.

[0007] Preferably, the total thickness of the SiOx-TCO-SiNx gradient refractive thin film layer is 90-120 nm.

[0008] Preferably, the SiOx film has a thickness of 5-10 nm, the TCO film has a thickness of 70-60 nm, and the SiNx film has a thickness of 22-30 nm.

[0009] Preferably, the SiNx thin film has a refractive index of 2.1, the TCO thin film has a refractive index of 1.9, and the SiOx thin film has a refractive index of 1.45.

[0010] Preferably, a metallized electrode is further disposed on the SiOx-TCO-SiNx gradient refractive thin film layer, wherein the metallized electrode is any one of the following: the second silver-copper electrode exactly covers the first nano-silver powder electrode, the second silver-copper electrode partially covers the first nano-silver powder electrode, or the second silver-copper electrode extends beyond the first nano-silver powder electrode.

[0011] Preferably, the metallized electrode is formed by screen printing, through screen printing mark design and alignment, to create a composite metallized pattern.

[0012] Preferably, the metallized electrode is cured after printing at a temperature of 200°C for 5 minutes.

[0013] The second aspect provides a method for resisting degradation of heterojunction solar cells, wherein the method includes SiOx-TCO-SiNx gradient refractive thin film layers disposed on both the backlight side and the light-receiving side of a monocrystalline silicon wafer substrate. The SiOx-TCO-SiNx gradient refractive thin film layer comprises, inwardly from the light-receiving surface, a SiNx thin film, a TCO thin film, and a SiOx thin film; Furthermore, the refractive index gradients of SiNx films, TCO films, and SiOx films decrease.

[0014] A third aspect provides a method for fabricating a heterojunction solar cell, the method comprising: 1) Silicon wafer gettering; 2) Down-making and cleaning; 3) Amorphous silicon deposition; 4) Gradient refractive index film deposition: A SiOx film is uniformly deposited on the surface of amorphous / microcrystalline silicon using PECVD or ALD methods; a high-transmittance TCO film is uniformly deposited on the surface of the SiOx film using PVD or RPD methods; and a SiNx film is uniformly deposited on the surface of the TCO film using PECVD methods; wherein the refractive index gradient of the SiNx film, TCO film and SiOx film decreases. 5) Metallization: Using screen printing technology, composite metallization patterns are formed on both sides of the silicon wafer through two printing processes; 6) Curing.

[0015] Preferably, the SiNx thin film has a refractive index of 2.1, the TCO thin film has a refractive index of 1.9, and the SiOx thin film has a refractive index of 1.45.

[0016] (III) Beneficial Effects This invention provides a heterojunction battery structure and an anti-degradation method, which has the following advantages compared with the prior art: To improve the conversion efficiency and weather resistance of heterojunction (HJT) solar cells, a composite gradient refractive thin film is added to increase light absorption and isolate moisture, reducing the impact of moisture on the TCO film layer, thereby improving the reliability of HJT cells. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the heterojunction battery structure of the present invention; Figure 2 This is a schematic diagram of the first type of metallized electrode; Figure 3 This is a schematic diagram of the second type of metallized electrode; Figure 4 This is a schematic diagram of the third type of metallized electrode; Among them, the single crystal silicon wafer substrate 1, the first intrinsic amorphous thin film layer 2-1, the first doped amorphous / microcrystalline thin film layer 3-1, the first SiOx-TCO-SiNx gradient refractive thin film layer 4-1, and the metallized electrode 5. Second intrinsic amorphous thin film layer 2-2, second doped amorphous / microcrystalline thin film layer 3-2, second SiOx-TCO-SiNx gradient refractive thin film layer 4-2. Specific implementation methods To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Traditional heterojunction solar cells suffer from weak adhesion between the TCO film layer and the traditional encapsulating film, and are sensitive to moisture, leading to poor module reliability and susceptibility to white spots and delamination. To address these issues, the industry currently improves the encapsulating film materials; however, this significantly increases the cost of the module encapsulating film.

[0018] Based on this, embodiments of the present invention provide a heterojunction battery structure, the battery structure including SiOx-TCO-SiNx gradient refractive thin film layers disposed on both the backlight surface and the light-receiving surface of a monocrystalline silicon wafer substrate. The SiOx-TCO-SiNx gradient refractive thin film layer comprises, inwardly from the light-receiving surface, a SiNx thin film, a TCO thin film, and a SiOx thin film; Furthermore, the refractive indices of the SiNx thin film, TCO thin film, and SiOx thin film decrease in that order.

[0019] Specifically, the battery structure described in the implementation process includes a monocrystalline silicon wafer substrate, an intrinsic amorphous thin film layer, a doped amorphous / microcrystalline thin film layer, a SiOx-TCO-SiNx gradient refractive thin film layer, and a metallized electrode.

[0020] The intrinsic amorphous silicon thin film layer comprises one or more amorphous silicon composite thin film layers. The amorphous / microcrystalline thin film layer comprises one amorphous / microcrystalline thin film layer or multiple amorphous / microcrystalline composite thin film layers; The SiOx-TCO-SiNx gradient refractive thin film layer comprises a composite film layer composed of SiOx, TCO and SiNx thin films with different refractive indices; The metallized electrode comprises a first nano-silver powder electrode layer and a second silver-copper electrode layer; Specifically, such as Figure 1 As shown, a first intrinsic amorphous thin film layer 2-1 is disposed on the light-receiving surface of the single crystal silicon wafer substrate 1, a first doped amorphous / microcrystalline thin film layer 3-1 is disposed on the first intrinsic amorphous thin film layer 2-1, a first SiOx-TCO-SiNx gradient refractive thin film layer 4-1 is disposed on the first doped amorphous / microcrystalline thin film layer 3-1, and a metallized electrode 5 is disposed on the surface of the first SiOx-TCO-SiNx gradient refractive thin film layer 4-1; A second intrinsic amorphous thin film layer 2-2 is disposed on the backlight surface of the single crystal silicon wafer substrate 1. A second intrinsically doped amorphous / microcrystalline thin film layer 3-2 is disposed on the second intrinsically doped amorphous / microcrystalline thin film layer 3-2. A second intrinsic SiOx-TCO-SiNx gradient refractive thin film layer 4-2 is disposed on the second intrinsically doped amorphous / microcrystalline thin film layer 3-2. A metallized electrode 5 is disposed on the surface of the second intrinsic SiOx-TCO-SiNx gradient refractive thin film layer 4-2. The metallization electrode 5 utilizes screen printing technology, employing two printing overlays and screen mark point design and alignment to form a composite metallization pattern on both sides of the silicon wafer; the specific structure is as follows. Figure 2-4 As shown; specifically, as Figure 2 As shown, the second silver-copper electrode can precisely cover the first nano-silver powder electrode; or, as... Figure 3 As shown, it can also partially cover the first silver nanopowder electrode; or, as... Figure 4 As shown, it can also exceed the first nanometer silver powder electrode; The metallized electrodes are printed and then cured at 200°C for 5 minutes. This temperature and time process cures the solar cell, forming a good ohmic contact.

[0021] Furthermore, the embodiments of the present invention also provide the above-mentioned heterojunction solar cell fabrication process. 1) Silicon wafer gettering: By using tubular P-source diffusion or chain coating followed by high-temperature diffusion, the impurity content in the silicon wafer is reduced, and the minority carrier recombination centers in the silicon wafer are reduced. 2) Texturing and cleaning: The silicon wafer is anisotropically etched with an alkaline solution to obtain a pyramid-shaped textured surface; the textured surface is smoothed and RCA cleaning is used to achieve a high degree of cleanliness on the silicon wafer surface. 3) Amorphous silicon deposition: Through plasma-enhanced chemical vapor deposition, intrinsic amorphous silicon thin films with passivation effect and doped amorphous / microcrystalline silicon films are deposited on both sides of a silicon wafer to form a pn junction; 4) Gradient Refractive Index Thin Film Deposition: A SiOx thin film is uniformly deposited on the surface of amorphous / microcrystalline silicon using PECVD or ALD methods to suppress interfacial carrier recombination, reduce contact resistance, and protect the microcrystalline structure. A high-transmittance TCO thin film is uniformly deposited on the surface of the SiOx thin film using PVD or RPD methods. A SiNx thin film is uniformly deposited on the outer layer of the TCO thin film using PECVD or other methods. Specifically, from the light-receiving surface inwards, the layers are SiNx, TCO, and SiOx. The total thickness is 90-120 nm. The refractive index of the SiNx film is 2.1, the refractive index of the TCO film is 1.9, and the refractive index of the SiOx film is 1.45. This composite gradient refractive index thin film reduces the reflectivity of the battery. 5) Metallization: Using screen printing technology, composite metallization patterns are formed on both sides of the silicon wafer through two printing processes; using screen printing, composite metallization patterns are formed through screen mark point design and precise alignment. 6) Curing: The solar cell is cured at a certain temperature and time to form a good ohmic contact; the curing temperature is 200℃ and the time is 5min; by controlling the temperature and time, the nano silver powder melts and penetrates the SiNx film to form a good ohmic contact with the TCO film.

[0022] 7) Photoinjection regeneration: Using photoinjection technology, reduce a-Si:H / c-Si interface recombination and improve TCO and Ag contact; This invention provides a heterojunction solar cell, aiming to improve the conversion efficiency and weather resistance of the heterojunction solar cell module. By adding a composite gradient refractive film layer, light absorption is increased and water vapor is isolated, reducing the impact of water vapor on the TCO film layer, thereby improving the reliability of the HJT solar cell.

[0023] Furthermore, in the metallization electrode process, a low-temperature conductive paste made of nano-silver powder is used. Taking advantage of the low melting point of the nano-silver powder particles, it solidifies and melts at low temperatures, forming a good ohmic contact with the silicon wafer. This allows for the fabrication of heterojunction cells while effectively avoiding damage to the passivation layer of the solar cell itself at high temperatures. Conventional low-temperature silver paste powder is micron-sized with a melting temperature of 700-900℃. The nano-silver powder low-temperature conductive paste refers to a material that uses nano-silver powder as a conductive filler, can solidify at lower temperatures, and possesses good conductivity. Using nano-silver powder can significantly reduce the curing temperature of the paste.

[0024] The industry currently uses improved encapsulation film materials to solve problems such as weak adhesion between the TCO film layer on the surface of heterojunction cells and traditional encapsulation films, but the drawback is that the cost of the module encapsulation film increases significantly.

[0025] This invention provides a method for resisting degradation of heterojunction solar cells, wherein the method includes forming SiOx-TCO-SiNx gradient refractive thin film layers on both the backlight side and the light-receiving side of a monocrystalline silicon wafer substrate. The SiOx-TCO-SiNx gradient refractive thin film layer comprises, inwardly from the light-receiving surface, a SiNx thin film, a TCO thin film, and a SiOx thin film; Furthermore, the refractive index gradients of SiNx films, TCO films, and SiOx films decrease.

[0026] After preparing a moisture-resistant thin film on the surface of the solar cell, this film can isolate the TCO film layer, reduce the contact between the TCO film layer and moisture, and improve the adhesion of the adhesive film, thereby improving the weather resistance of this type of module. The SiOx-TCO-SiNx gradient refractive thin film layer increases light absorption and isolates moisture, reducing the impact of moisture on the TCO film layer, thus improving the reliability of the HJT cell.

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail.

[0028] Comparative example: For N-type with a thickness of 110 μm and a resistivity of 1 Ω P-diffusion gettering was performed on a single-crystal silicon wafer of cm at a gettering temperature of 800℃. The silicon wafer is texturized and cleaned to form a pyramidal textured surface, which removes impurity ions and cleans the surface. The textured surface size is 1μm. Bi-intrinsic amorphous silicon layers and doped amorphous / microcrystalline silicon layers on the front and back sides were prepared by plasma chemical vapor deposition. The thickness of the intrinsic amorphous silicon on the front and back sides was 8 nm, the thickness of the P-type amorphous / microcrystalline silicon was 15 nm, and the thickness of the N-type amorphous / microcrystalline silicon was 20 nm. ITO thin films were deposited by magnetron sputtering, with a thickness of 100 nm on both the front and back sides. A mask design was used to support silicon wafers on a carrier plate, with a mask width of 1 mm. The front and back silver metal electrodes are formed using screen printing technology. A main grid-free design is adopted. The back silver metal electrode has a sub-grid linewidth of 0.030mm and 114 sub-grids. The front silver metal electrode has a sub-grid linewidth of 0.030mm and 84 sub-grids.

[0029] The silicon wafer was cured at a temperature of 200℃ for 15 minutes. The solar cells were subjected to light injection treatment at a temperature of 200℃ for 20 seconds. Test the battery's electrical performance and monitor cell degradation; Example 1: For N-type with a thickness of 110 μm and a resistivity of 1 Ω P-diffusion gettering was performed on a single-crystal silicon wafer of cm at a gettering temperature of 800℃. The silicon wafer is texturized and cleaned to form a pyramidal textured surface, which removes impurity ions and cleans the surface. The textured surface size is 1μm. Bi-intrinsic amorphous silicon layers and doped amorphous / microcrystalline silicon layers on the front and back sides were prepared by plasma chemical vapor deposition. The thickness of the intrinsic amorphous silicon on the front and back sides was 8 nm, the thickness of the P-type amorphous / microcrystalline silicon was 15 nm, and the thickness of the N-type amorphous / microcrystalline silicon was 20 nm. SiOx thin films were prepared by plasma chemical vapor deposition with a thickness of 5 nm on both sides. ITO thin films were prepared by magnetron sputtering with a thickness of 70 nm on both sides. A mask design was used to support silicon wafers on a carrier to form a mask with a width of 1 mm. SiNx thin films were prepared by plasma chemical vapor deposition with a thickness of 25 nm on both sides. The front and back silver metal electrodes are formed by screen printing technology through a two-stage printing process. The design adopts a gridless design. The back silver metal electrode has a sub-grid linewidth of 0.025mm and 114 sub-grids. The front silver metal electrode has a sub-grid linewidth of 0.025mm and 84 sub-grids. The silicon wafer is cured at a temperature of 200℃ for 5 minutes. The solar cells were subjected to light injection treatment at a temperature of 200℃ for 20 seconds. Test the battery's electrical performance and monitor cell degradation; Example 2: For N-type with a thickness of 110 μm and a resistivity of 1 Ω P-diffusion gettering was performed on a single-crystal silicon wafer of cm at a gettering temperature of 800℃. The silicon wafer is texturized and cleaned to form a pyramidal textured surface, which removes impurity ions and cleans the surface. The textured surface size is 1μm. Bi-intrinsic amorphous silicon layers and doped amorphous / microcrystalline silicon layers on the front and back sides were prepared by plasma chemical vapor deposition. The thickness of the intrinsic amorphous silicon on the front and back sides was 8 nm, the thickness of the P-type amorphous / microcrystalline silicon was 15 nm, and the thickness of the N-type amorphous / microcrystalline silicon was 20 nm. SiOx thin films were prepared by plasma chemical vapor deposition with a thickness of 8 nm on both sides. ITO thin films were prepared by magnetron sputtering with a thickness of 70 nm on both sides. A mask design was used to support silicon wafers on a carrier to form a mask with a width of 1 mm. SiNx thin films were prepared by plasma chemical vapor deposition with a thickness of 22 nm on both sides. The front and back silver metal electrodes are formed by screen printing technology through a two-stage printing process. The design adopts a gridless design. The back silver metal electrode has a sub-grid linewidth of 0.025mm and 114 sub-grids. The front silver metal electrode has a sub-grid linewidth of 0.025mm and 84 sub-grids. The silicon wafer is cured at a temperature of 200℃ for 5 minutes. The solar cells were subjected to light injection treatment at a temperature of 200℃ for 20 seconds. Test the battery's electrical performance and monitor cell degradation; Example 3: For N-type with a thickness of 110 μm and a resistivity of 1 Ω P-diffusion gettering was performed on a single-crystal silicon wafer of cm at a gettering temperature of 800℃. The silicon wafer is texturized and cleaned to form a pyramidal textured surface, which removes impurity ions and cleans the surface. The textured surface size is 1μm. Bi-intrinsic amorphous silicon layers and doped amorphous / microcrystalline silicon layers on the front and back sides were prepared by plasma chemical vapor deposition. The thickness of the intrinsic amorphous silicon on the front and back sides was 8 nm, the thickness of the P-type amorphous / microcrystalline silicon was 15 nm, and the thickness of the N-type amorphous / microcrystalline silicon was 20 nm. SiOx thin films were prepared by plasma chemical vapor deposition with a thickness of 10 nm on both sides. ITO thin films were prepared by magnetron sputtering with a thickness of 60 nm on both sides. A mask design was used to support silicon wafers on a carrier to form a mask with a width of 1 mm. SiNx thin films were prepared by plasma chemical vapor deposition with a thickness of 30 nm on both sides. The front and back silver metal electrodes are formed by screen printing technology through a two-stage printing process. The design adopts a gridless design. The back silver metal electrode has a sub-grid linewidth of 0.025mm and 114 sub-grids. The front silver metal electrode has a sub-grid linewidth of 0.025mm and 84 sub-grids. The silicon wafer is cured at a temperature of 200℃ for 5 minutes. The solar cells were subjected to light injection treatment at a temperature of 200℃ for 20 seconds. The electrical performance and efficiency of the solar cells are tested according to the national standard GB / T 6495.1, and the cell degradation is monitored. The data comparison results are as follows: The data results from the comparative examples and embodiments above demonstrate that the technology of this invention can effectively improve the conversion efficiency of HJT batteries, reduce HJT battery degradation, and enhance the weather resistance of HJT battery modules.

[0030] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0031] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A heterojunction battery structure, characterized in that, The battery structure includes SiOx-TCO-SiNx gradient refractive thin film layers disposed on both the backlight side and the light-receiving side of the monocrystalline silicon wafer substrate. The SiOx-TCO-SiNx gradient refractive thin film layer comprises, inwardly from the light-receiving surface, a SiNx thin film, a TCO thin film, and a SiOx thin film; Furthermore, the refractive indices of the SiNx thin film, TCO thin film, and SiOx thin film decrease in that order.

2. The heterojunction battery structure according to claim 1, characterized in that, The total thickness of the SiOx-TCO-SiNx gradient refractive thin film layer is 90-120 nm.

3. The heterojunction battery structure according to claim 1, characterized in that, The SiOx film has a thickness of 5-10 nm, the TCO film has a thickness of 70-60 nm, and the SiNx film has a thickness of 22-30 nm.

4. The heterojunction battery structure according to claim 1, characterized in that, The SiNx thin film has a refractive index of 2.1, the TCO thin film has a refractive index of 1.9, and the SiOx thin film has a refractive index of 1.

45.

5. The heterojunction battery structure according to claim 1, characterized in that, The SiOx-TCO-SiNx gradient refractive thin film layer is further provided with a metallized electrode, wherein the metallized electrode is any one of the following: the second silver-copper electrode exactly covers the first nano-silver powder electrode, the second silver-copper electrode partially covers the first nano-silver powder electrode, or the second silver-copper electrode extends beyond the first nano-silver powder electrode.

6. The heterojunction battery structure according to claim 5, characterized in that, The metallized electrode is formed by screen printing, through the design and alignment of screen mark points, to create a composite metallized pattern.

7. The heterojunction battery structure according to claim 6, characterized in that, The metallized electrode is printed and then cured at a temperature of 200°C for 5 minutes.

8. A method for resisting degradation in heterojunction solar cells, characterized in that, The anti-attenuation method includes SiOx-TCO-SiNx gradient refractive thin film layers disposed on both the backlight side and the light-receiving side of the monocrystalline silicon wafer substrate. The SiOx-TCO-SiNx gradient refractive thin film layer comprises, inwardly from the light-receiving surface, a SiNx thin film, a TCO thin film, and a SiOx thin film; Furthermore, the refractive index gradients of SiNx films, TCO films, and SiOx films decrease.

9. A method for fabricating a heterojunction solar cell, characterized in that, The preparation method includes: 1) Silicon wafer gettering; 2) Down-making and cleaning; 3) Amorphous silicon deposition; 4) Gradient refractive index film deposition: A SiOx film is uniformly deposited on the surface of amorphous / microcrystalline silicon using PECVD or ALD methods; a high-transmittance TCO film is uniformly deposited on the surface of the SiOx film using PVD or RPD methods; and a SiNx film is uniformly deposited on the surface of the TCO film using PECVD methods; wherein the refractive index gradient of the SiNx film, TCO film and SiOx film decreases. 5) Metallization: Using screen printing technology, composite metallization patterns are formed on both sides of the silicon wafer through two printing processes; 6) Curing.

10. The method for fabricating a heterojunction solar cell according to claim 9, characterized in that, The SiNx thin film has a refractive index of 2.1 The TCO thin film has a refractive index of 1.9 and the SiOx has a refractive index of 1.45.

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