A silicon nitride film layer on the back of a battery, a PERC battery and a preparation method thereof

Abstract

The present invention discloses a silicon nitride film layer on the back of a battery, a PERC battery and a preparation method, belonging to the field of single-crystal PERC battery manufacturing. Aiming at the problems existing in the prior art, such as the low reflectivity of the back film of the battery, the low conversion efficiency of the battery, and the high temperature requirement, high energy consumption and high cost of the back film manufacturing process, the present invention provides a PERC battery. The battery includes a front silicon nitride film layer, a back aluminum oxide film layer and a back silicon nitride film layer. The back silicon nitride film layer has a five-layer structure. When manufacturing the battery, the back aluminum oxide film layer is deposited first, then the front silicon nitride film layer is deposited, and finally the back silicon nitride film layer is deposited. Using a low-temperature manufacturing process not only improves the photoelectric conversion efficiency of the battery chip and the performance of the single-crystal PERC battery product, but also reduces the energy consumption during the manufacturing process, shortens the manufacturing time, improves the production efficiency, and is suitable for wide application.

Description

Technical Field

[0001] The present invention relates to the field of single-crystal PERC cell manufacturing, and more specifically, to a silicon nitride film layer on the back surface of a cell, a PERC cell, and a preparation method thereof. Background Art

[0002] The current mainstream technology for crystalline silicon solar cells is PERC (Passivated Emitter Rear Cell) - passivated emitter and rear cell technology, which improves the conversion efficiency by adding a dielectric passivation layer on the rear side of the cell. The PERC cell implements passivation technology on the back surface of the cell, enhances the internal back reflection of light in the silicon substrate, reduces the back surface recombination, maximizes the potential gradient across the P-N junction, enables electrons to flow more stably, reduces electron recombination, and thus effectively improves the efficiency of the PERC cell.

[0003] Covering the back surface of a single-crystal PREC cell with aluminum oxide and silicon nitride is the core technology. Since the aluminum oxide passivation layer deposited on the silicon substrate is relatively thin, it is necessary to deposit silicon nitride on this aluminum oxide film for protection, while increasing the reflectivity of the back surface, aiming to obtain a better passivated surface, thereby improving the long-wave response, so as to improve Isc / Uoc and thus enhance the conversion efficiency of the cell.

[0004] The film formation quality of the back coating film has a relatively important impact on the passivation effect of the back surface of the cell and the protection of aluminum oxide. During the deposition of silicon nitride by tube PECVD, as the temperature rises, the hydroxyl groups in the aluminum oxide film decompose, the H content in the film decreases, the passivation effect on the cell surface weakens, and the effective minority carrier lifetime decreases. Selecting an appropriate growth temperature can make the crystallization degree of the film reach the best. The existing back films are mainly single-layer and three-layer films, and the back surface reflection effect is the bottleneck, the short-circuit current cannot increase, and at the same time, the process temperature of the back film is about 530 degrees, with high energy consumption and high cost.

[0005] Chinese Patent Application, Application No. CN 201910067978.2, Publication Date: May 21, 2019, discloses an anti-PID double-sided PERC cell's back surface antireflection passivation film and a double-sided PERC cell and its preparation method. The back surface antireflection passivation film includes a SiO2 layer; an alumina layer in contact with the SiO2 layer; an n-layer SiNx in contact with the alumina layer, where 1 ≤ x ≤ 1.5; from the inside outwards, the refractive index of the n-layer SiNx shows a decreasing trend; the thickness of the n-layer SiNx shows an increasing trend; n is a positive integer. In the present invention, by providing a SiO2 layer and an n-layer SiNx on the back surface of the silicon wafer, with the refractive index of the n-layer SiNx showing a decreasing trend and the thickness of the n-layer SiNx showing an increasing trend, the passivation film has a better anti-PID effect. For the double-sided PERC cell under the test conditions of -1500V, 85% humidity, and 85°C, after 96h, the front surface attenuation ratio is 0.34% and the back surface attenuation ratio is 0.66%; after 192h, the front surface attenuation ratio is 1.65% and the back surface attenuation ratio is 2.10%. In this invention, the silicon-nitrogen ratio gradually decreases, that is, starting from the base silicon layer, from the inside outwards, the refractive index of the film layer gradually decreases and the film layer thickness gradually increases. This film layer structure has relatively poor wavelength response matching. Summary of the Invention

[0006] 1. Technical Problems to be Solved

[0007] Aiming at the problems existing in the prior art, such as the low reflectivity of the back film of the prior art battery, low battery conversion efficiency, high temperature requirements for the back film manufacturing process, high energy consumption and high cost, the present invention provides a silicon nitride film layer on the back of the battery, a PERC battery and a preparation method, which can improve the photoelectric conversion efficiency of the battery chip and enhance the performance of the single crystal PERC battery product.

[0008] 2. Technical Solutions

[0009] The object of the present invention is achieved through the following technical solutions.

[0010] A silicon nitride film layer on the back of the battery includes a five-layer structure. The thicknesses of the five-layer silicon nitride film layer on the back are α1, α2, α3, α4, and α5 from the inside outwards, and the thicknesses of α1 to α5 satisfy α5 ≥ α3 ≥ α1 ≥ α4 ≥ α2 or α5 ≥ α3 ≥ α1 ≥ α2 ≥ α4; the refractive indices of the five-layer silicon nitride film layer on the back are β1, β2, β3, β4, and β5 from the inside outwards, and the refractive indices of β1 to β5 satisfy β1 ≥ β3 > β4 > β2 = β5. The present invention designs a five-layer structure for the silicon nitride film layer on the back of the battery to achieve the best battery conversion efficiency. Compared with the traditional film layer where the thickness gradually increases and the refractive index gradually decreases, the film layer of the present invention has better wavelength response matching and obvious current advantages.

[0011] Further, the thicknesses of the five-layer backside silicon nitride film layer range from inside to outside as 25nm ± 5nm, 10nm ± 5nm, 30nm ± 5nm, 20nm ± 5nm, and 45nm ± 5nm respectively, and the refractive indices of the five-layer backside silicon nitride film layer range from inside to outside as 2.6 ± 0.2, 2.1 ± 0.1, 2.5 ± 0.1, 2.3 ± 0.05, and 2.1 ± 0.1 respectively.

[0012] Further, the thicknesses of the five-layer backside silicon nitride film layer are 25nm, 10nm, 30nm, 20nm, and 45nm respectively from inside to outside, and the refractive indices of the five-layer backside silicon nitride film layer are 2.6, 2.1, 2.5, 2.3, and 2.1 respectively from inside to outside.

[0013] A PERC cell includes the above-mentioned backside silicon nitride film layer of the cell, and also includes a silicon wafer and a backside aluminum oxide film layer disposed on the backside of the silicon wafer. The backside aluminum oxide film layer is located between the silicon wafer and the backside silicon nitride film layer. Depositing the backside silicon nitride film layer on the backside aluminum oxide film layer can protect the backside aluminum oxide film layer, increase the reflectivity of the backside at the same time, obtain a better passivation surface, and improve the conversion efficiency of the cell.

[0014] Further, the cell also includes a front-side silicon nitride film layer disposed on the front side of the silicon wafer. The front-side silicon nitride film layer is a reflection-reducing passivation film.

[0015] A method for manufacturing a PERC cell includes the following steps:

[0016] Step 1: Obtain a pretreated silicon wafer through operations of texturing, diffusion, first annealing, etching, and second annealing;

[0017] Step 2: Deposit a backside aluminum oxide film layer on the backside of the pretreated silicon wafer;

[0018] Step 3: Deposit a front-side silicon nitride film layer on the front side of the pretreated silicon wafer;

[0019] Step 4: Deposit a backside silicon nitride film layer on the backside aluminum oxide film layer;

[0020] Step 5: Use a laser to groove the backside, and form a back electrode, a back electric field, and a front electrode by screen printing; sinter, and the manufacturing is completed.

[0021] In the manufacturing of the cell of the present invention, the front film is made first and then the back film. When the backside aluminum oxide is transported through an automated belt and wiped by a graphite boat without coating, the high-frequency physical contact may increase the EL defect. Through experiments by the equipment and automation manufacturers, the EL defect is at the same level as the process of making the back film first and then the front film.

[0022] The present invention changes the coating sequence of the original coating process from back-to-back positive to back-to-positive-back. By changing the coating sequence, the annealing time of the backside aluminum oxide is prolonged, the negative charge density of the backside aluminum oxide is increased, the field passivation effect of the aluminum oxide is enhanced, and the formed Al2O3 contains a high density of fixed negative charges, and the formed electric field can reduce the surface recombination. Without any additional investment, the conversion efficiency of the battery cells of the present invention is increased by 0.05%-0.07%.

[0023] Furthermore, in step 4, the process temperature for depositing the backside silicon nitride film layer is 470°C ± 20°C. The present invention studies the influence of different back film temperature conditions on the efficiency of battery cells, and at the same time studies the matching of different temperatures with different back film layer structures to further improve the photoelectric conversion efficiency of battery cells, confirms the process matching of low-temperature back film and five-layer back film, and expands a new direction for the optimization of the back film process; the new matching process has a significant improvement in the photoelectric conversion efficiency compared with the conventional manufacturing process.

[0024] After backside passivation, annealing is carried out at 500°C. Tetrahedrons are significantly superior to octahedrons in terms of density, and more negative charges can be obtained, enhancing the backside passivation effect and facilitating the release of H2 at the same time. The low-temperature manufacturing process shortens the annealing time of the back film and improves the production efficiency.

[0025] Furthermore, in step 4, the backside silicon nitride film layer is deposited by plasma-enhanced chemical vapor deposition.

[0026] Furthermore, the plasma-enhanced chemical vapor deposition is tube-type plasma-enhanced chemical vapor deposition.

[0027] Furthermore, in step 2, the front-side silicon nitride film layer is deposited by plasma-enhanced chemical vapor deposition. During production, the competitiveness of the battery is enhanced by improving the compactness of the backside aluminum oxide.

[0028] Furthermore, in step 3, the backside aluminum oxide film layer is formed by atomic layer deposition.

[0029] Furthermore, in step 3, the backside silicon nitride film layer is provided with raw materials by gases such as NH3 and SiH4, and the refractive indexes of each layer of the five-layer film are controlled by adjusting the flow ratio of NH3 and SiH4 during production.

[0030] The back film five-layer battery of the present invention with low-temperature production first on the front side and then on the back side, compared with the back film three-layer battery of the existing production line, without any additional investment, the conversion efficiency is increased by 0.05%-0.07%, and related parameters such as the open-circuit voltage Uoc, short-circuit current Isc, and fill factor FF are also significantly improved. The low-temperature manufacturing process not only has better battery conversion efficiency, but also reduces the annealing time of the back film and improves the production efficiency, and is suitable for wide application.

[0031] 3. Beneficial effects

[0032] Compared with the prior art, the advantages of the present invention are as follows:

[0033] (1) For the five-layer silicon nitride film layer of the back film of the present invention, the refractive index from the inside to the outside is high-low-high-low-low, and the thickness of the film layer from the inside to the outside is thick-thin-thick-thin-thick. Compared with the traditional film layer whose thickness gradually increases and refractive index gradually decreases, the five-layer silicon nitride film layer on the back of the present invention has better wavelength matching, better efficiency performance, and obvious current performance advantages.

[0034] (2) For the PERC solar cell of the present invention, the coating sequence of the front film first and then the back film is used, and a low-temperature process is adopted. The process temperature for depositing the back silicon nitride film layer is 470°C ± 20°C. The five-layer structure of the back film is beneficial to the back contact and the surface passivation effect, improving the open-circuit voltage. At the same time, a gradient mirror surface is formed on the back, increasing the internal reflection of long waves, enhancing the reflectivity of the back, improving the minority carrier lifetime of the battery, increasing the short-circuit current, and thus improving the photoelectric conversion efficiency. By using the coating process of front first and back later, without any additional investment, the conversion efficiency of the battery slices of the present invention is increased by 0.05%-0.07%, and the proportion of defective products with black spots is reduced by about 0.15%.

[0035] The manufacturing method of the present invention can realize the preparation of the five-layer film of the back film under low-temperature conditions on different PECVD deposition devices. The process steps are simple, the process is stable and easy to control; the process temperature of the back film is 50 degrees lower than the conventional process, and the energy consumption is significantly reduced. The low-temperature manufacturing process compresses the annealing time of the back film by 3 minutes, increases the production capacity by 40,000 pieces / shift, and improves the production efficiency. Description of the Drawings

[0036] Figure 1 It is a schematic diagram of the battery structure of the present invention;

[0037] Figure 2 It is a schematic diagram of the five-layer structure of the back film of the battery of the present invention. Detailed Embodiments

[0038] The present invention will be described in detail below in conjunction with the drawings in the specification and specific embodiments.

[0039] Example 1

[0040] Taking P-type monocrystalline silicon as an example, this embodiment describes the five-layer structure of the back film of its PERC solar cell and the corresponding low-temperature preparation method.

[0041] First, the battery structure of the present invention is as Figure 1As shown, the front side of the single-crystal cell is coated with a front-side silicon nitride film layer to reduce reflection. The back film of the cell includes a back-side aluminum oxide film layer and a back-side silicon nitride film layer. The back-side silicon nitride film layer protects the back-side aluminum oxide film layer and at the same time increases the reflectivity of the back side, obtaining a better passivated surface and improving the conversion efficiency of the cell.

[0042] Table 1

[0043] Backside silicon nitride film layer Film thickness Refractive index First layer 25nm ± 5nm 2.6±0.2 Second layer 10nm ± 5nm 2.1±0.1 Third layer 30nm ± 5nm 2.5±0.1 Fourth layer 20nm ± 5nm 2.3±0.05 Fifth layer 45nm ± 5nm 2.1±0.1

[0044] As Figure 2 shown, the back-side silicon nitride film layer in this embodiment is a five-layer film. The five-layer silicon nitride film layer is made using NH3 and SiH4 special gas cabinets. During production, the refractive index of each layer of the five-layer film is controlled by adjusting the flow ratio of NH3 and SiH4. The thicknesses of the five-layer back-side silicon nitride film layer from the inside out are α1, α2, α3, α4, and α5 respectively, and the thicknesses of α1 to α5 are α5≥α3≥α1≥α4≥α2 or α5≥α3≥α1≥α2≥α4. Compared with the first, third, and fifth layers, the thicknesses of the second and fourth layers in the five-layer film are slightly lower, showing a thick-thin-thick-thin-thick state. That is to say, among the five-layer film, the first, third, and fifth layers are all higher than the second layer; or the first, third, and fifth layers are all higher than the fourth layer. The refractive indices of the five-layer back-side silicon nitride film layer from the inside out are β1, β2, β3, β4, and β5 respectively, and the refractive indices of β1 to β5 are β1≥β3>β4>β2=β5. The refractive index size distribution of the five-layer film is high-low-high-low-low. In this embodiment, the thickness and refractive index of the five-layer film are shown in Table 1. The fixed values in Table 1 are the average values measured during the experiment process. Based on the measured average values, there are corresponding ranges for the thickness and refractive index values of the five-layer film.

[0045] The back-side silicon nitride film layer constructs a five-layer film structure by adjusting the different flow ratios of silane or ammonia. It is optimized based on the single-layer film, double-layer film and other structures in the prior art. By setting the different thicknesses and refractive index data of the five-layer film structure, the deposition passivation effect is increased, the open-circuit voltage Uoc and short-circuit current Isc of the cell are improved, and thus the conversion efficiency of the cell is improved. The five-layer structure of the back film is combined with a 470°C low-temperature deposition process. Through the testing of the cell performance, it is verified that the conversion efficiency of the cell is significantly improved.

[0046] According to the thickness and refractive index of the five-layer film of the back-side silicon nitride film layer shown in the parameters of Table 1, the refractive index from the first layer to the fifth layer from the inside out is high-low-high-low-low, and the thickness of the film layer is thick-thin-thick-thin-thick. After finite experimental data analysis, the film layer with the thickness and refractive index described in the present invention has qualified PID compared with the film layer with gradually decreasing refractive index and gradually increasing film layer thickness, has a good wavelength matching, and shows better efficiency and obvious current advantages.

[0047] Example 2

[0048] For the back film five - layer structure battery described in Example 1, the specific manufacturing steps are as follows:

[0049] Step Ⅰ: Through the operations of texturing, diffusion, first annealing, etching, and second annealing, a pre - treated silicon wafer is obtained;

[0050] a), Texturing:

[0051] Isotropic etching is carried out with an acid solution to obtain a surface structure with many pits on the surface, which can also play a good light - trapping role. The single - crystal silicon wafer is textured to form a pyramid - shaped texture, so that a uniform pyramid - shaped structure is formed on the surface of the silicon wafer, enhancing the light absorption and improving the short - circuit current and conversion efficiency of the battery.

[0052] The reaction equation is: Si + 4HNO3+6HF = H2SiF6 + 4NO3+4H2O.

[0053] b), Diffusion to prepare a PN junction:

[0054] The surface phosphorus source is advanced, and a PN junction is formed by using the thermal propulsion method. The gas carries the POCL3 solution into the diffusion furnace tube, and it reacts to generate phosphorus precipitation on the surface layer. Phosphorus penetrates into the interior of the silicon wafer at high temperature to form an N region.

[0055] The reaction equation is: 4POCL3 + 5O2 = 2P2O5+6CL2↑

[0056] 2P2O5 + 5Si = 4P↓+5SiO2

[0057] 4PCL5 + 5O2 = 2P2O5+10CL2↑

[0058] 4PCL3 + 5O2 = 2P2O5+6CL2↑

[0059] c), First annealing;

[0060] At about 450°C, the oxygen in silicon will be converted into oxygen donors, which will affect the correct measurement of resistivity, increase the resistivity of P - type single - crystal silicon, and even cause inversion; reduce the resistivity of N - type single - crystal silicon. Annealing treatment at about 700°C can make the oxygen donors return to the interstitial oxygen state and eliminate the influence of oxygen donors on resistivity measurement.

[0061] The reaction equation is: Si + O2 = SiO2 (high temperature)

[0062] d), Etching;

[0063] Backside polishing, etching, and removing phosphosilicate glass; using a mixed liquid of HNO3 and HF to etch the lower surface and edges of the diffused silicon wafer, removing the N-type silicon at the edges, and making the upper and lower surfaces of the silicon wafer insulated from each other.

[0064] e) Secondary annealing;

[0065] Similar to step c), the annealing treatment can make the oxygen donors return to the interstitial oxygen state and eliminate the influence of oxygen donors on the resistivity measurement.

[0066] The reaction equation is: Si + O2 = SiO2 (high temperature)

[0067] Step II: Deposit a backside aluminum oxide film layer on the backside of the pretreated silicon wafer;

[0068] ALD is Atomic Layer Deposition, a method that can deposit substances layer by layer in the form of a single atomic film on the surface of a substrate. Atomic layer deposition has similarities with ordinary chemical deposition.

[0069] Prepare a double-sided aluminum oxide film by ALD on the backside. It is a method of alternately pulsing gaseous precursors into the reactor and chemically adsorbing and reacting on the deposition substrate to form a deposition film. When the precursors reach the surface of the deposition substrate, they will chemically adsorb on its surface and undergo surface reactions.

[0070] The surface reaction of atomic layer deposition has self-limiting properties, namely the Chemisorption Self-Limitation (CS) and Sequential Reaction Self-Limitation (RS) processes. In fact, this self-limiting characteristic is precisely the basis of atomic layer deposition technology. Repeating this self-limiting reaction continuously forms the required thin film. There are a certain amount of AL-OH bonds in the Al2O3 thin film prepared by ALD. After annealing, the AL-OH bonds are transformed into AL-O bonds, and H atoms are released. Part of the H atoms enter the silicon surface to passivate the surface dangling bonds, and part of them polymerize to form H2.

[0071] Step III: Deposit a frontside silicon nitride film layer on the frontside of the pretreated silicon wafer;

[0072] Plasma Enhanced Chemical Vapor Deposition (PECVD) is a process that ionizes a gas containing atoms of the thin film components by means of microwaves or radio frequency, forming a plasma locally. The plasma has strong chemical activity and easily undergoes reactions to deposit the desired thin film on the substrate.

[0073] Using a low-temperature plasma as an energy source, the sample is placed on the cathode of a glow discharge under low pressure. The sample is heated to a predetermined temperature by glow discharge (or with an additional heating element), and then an appropriate amount of reaction gas is introduced. Through a series of chemical reactions and plasma reactions, a solid film is formed on the surface of the sample.

[0074] The reaction equation is: 4HN3 + 3SiH4 = Si3N4↓ + 12H2↑

[0075] Step IV: Deposit a back-side silicon nitride film layer on the back-side alumina film layer.

[0076] Based on the back-side alumina film in Step II, the front-side silicon nitride film in Step III, and the back-side silicon nitride film in Step IV, the silicon nitride film of the five-layer back-side film structure is made using NH3 and SiH4 special gas cabinets. During production, the refractive index of each layer of the five-layer film is controlled by adjusting the flow ratio of NH3 and SiH4. In this process, the low-temperature five-layer silicon nitride film process is matched and implemented at a temperature of 470°C ± 20°C.

[0077] During the deposition of silicon nitride by tube PECVD, as the temperature rises, the hydroxyl groups in the alumina film decompose, the H content in the film decreases, the surface passivation effect of the cell is weakened, and the effective minority carrier lifetime is reduced. Selecting an appropriate growth temperature can make the crystallization degree of the film reach the best. Different process temperature gradients are tried during this period, and the efficiency is the best at a temperature of 470°C.

[0078] In the ALD process, Al2O3 exists in two coordination forms: the octahedral center position of 6 nitrogen atoms and the tetrahedral center position of 4 oxygen atoms. After annealing heat treatment, the octahedral structure is transformed into a tetrahedral structure. The tetrahedron has an additional negative charge of oxygen, so the negative charge density of Al2O3 is increased. The coating sequence of the preparation process is changed from back-back-front to back-front-back, and the annealing time is increased, which is more conducive to the formation of tetrahedral alumina, thereby improving the passivation effect. First deposit the front film and then the back film, the cell conversion efficiency is increased by 0.05%, the cell reliability remains unchanged, the efficiency gain is large, and the cost reduction and efficiency increase are significant.

[0079] Step V: Use a laser to groove the back side, and screen-print to form the back electrode, back electric field, and front electrode; sinter, and the preparation is completed.

[0080] i). Locally groove the back side with a laser;

[0081] The current on the back surface of the silicon wafer is led out through the photolithography groove and absorbed by the back electric field, and then conducted to the back electric field.

[0082] ii). Screen-print to form the back electrode, back electric field, and front electrode. The front silver paste used for printing is an ozone single-sided polymerization paste;

[0083] Screen printing mainly uses the basic principle that the mesh holes in the graphic part of the screen plate are permeable to ink, while the mesh holes in the non-graphic part are impermeable to ink for printing.

[0084] For printing the front silver, single-sided single silver paste is used. In this experiment, taking the polymerized paste as an example, specifically, the etching performance of the ozone single-sided polymerized paste WL698 on alumina is used to reduce the thickness of the front alumina.

[0085] iii). Sintering enables the metal to form a good ohmic contact with silicon.

[0086] Test the electrical performance of the battery fabricated by the process of the present invention.

[0087] In the detection of the battery performance, three groups of experiments are designed in this embodiment to measure the parameters of the three-layer back film and the five-layer back film structures at different temperatures.

[0088] Experiment 1 is the temperature gradient experiment of the three-layer back film. The process temperatures are 450 °C, 470 °C, and 530 °C respectively, where 530 °C is the temperature of the existing production line. The experimental results are shown in Table 2.

[0089] Experiment 2 is the temperature gradient experiment of the five-layer back film. The process temperatures are 450 °C, 470 °C, and 530 °C respectively, where 530 °C is the temperature of the existing production line. The experimental results are shown in Table 3.

[0090] Experiment 3 is the experiment on the combination of the three-layer back film structure and the five-layer back film structure with low-temperature process quantity. The experimental results are shown in Table 4.

[0091] In Tables 2, 3, and 4, Eta represents the conversion efficiency of the battery. The battery conversion efficiency is an important parameter to measure the battery quality and technical level, and it is related to the battery structure, junction characteristics, material properties, working temperature, radioactive particle radiation damage, and environmental changes, etc. Uoc represents the open-circuit voltage. The open-circuit voltage is the output voltage value of the solar cell when it is placed under the illumination of a 100 mW / cm2 light source and the two ends are open. Isc represents the short-circuit current. The short-circuit current is the current flowing through both ends of the solar cell when the solar cell is placed under the illumination of a standard light source and the output end is short-circuited. Isc and Uoc characterize the conversion efficiency of the battery. FF represents the fill factor, also known as the curve factor. The fill factor refers to the ratio of the maximum power of the solar cell to the product of the open-circuit voltage and the short-circuit current, and it is an important parameter to evaluate the output characteristics of the solar cell. The higher the fill factor value, the closer the output characteristics of the solar cell are to a rectangle, and the higher the light conversion efficiency of the battery. Rser is the series resistance, Rshunt is the leakage parallel resistance, and Irevmax is the dark current.

[0092] Table 2

[0093]

[0094] Table 3

[0095]

[0096] Table 4

[0097]

[0098] According to the experimental data in Table 2 and Table 3, it can be obtained that the process efficiency is the highest at 470 °C, which is mainly manifested in the increase of the short-circuit current Isc. According to the experimental data in Table 4, the process efficiency of the back film five-layer film temperature gradient experiment is about 0.05% higher than that of the back film three-layer film battery conversion efficiency of the production line process, and parameters such as the open-circuit voltage Uoc, short-circuit battery Isc, and fill factor FF all have advantages.

[0099] The conversion efficiency of the battery refers to the ratio of the solar cell converting light energy into electrical energy. According to relevant literature records, the conversion rate of single-crystal batteries has increased from 19% in 2013 to nearly 20% in 2018. The back-side silicon nitride film layer battery with a five-layer structure prepared by the preparation method of the present invention at a temperature of 470 °C has been experimentally verified to have a conversion rate increase of about 0.05%, with a major breakthrough in the conversion rate and significant efficiency increase.

[0100] The production sequence of the battery of the present invention is: front process - back passivation - front film - back film - back process; changing the original coating sequence of back-back-front to back-front-back, by changing the coating sequence, the annealing time of the back-side aluminum oxide film layer is extended, the negative charge density of the back-side aluminum oxide film layer is increased, the field passivation effect of aluminum oxide is improved, and the formed Al2O3 contains a high density of fixed negative charges, and the formed electric field can reduce surface recombination.

[0101] The single-crystal PERC battery prepared by the preparation method of the present invention has an increased short-circuit current, an increased open voltage, and a battery conversion efficiency increased by 0.05% - 0.07% compared with the single-crystal PERC battery with a single-layer back-side PECVD film. It can be seen that under the condition of front-back for this single-crystal battery, using a five-layer film for the back film and matching with a low-temperature process, the efficiency increase is significant and the effect is better.

[0102] The present invention and its implementation manners are schematically described above. The description is not restrictive. Without departing from the spirit or basic features of the present invention, the present invention can be implemented in other specific forms. What is shown in the drawings is only one of the implementation manners of the present invention, and the actual structure is not limited thereto. Any reference signs in the claims should not limit the claimed claims. Therefore, if those of ordinary skill in the art are inspired by it and, without departing from the purpose of this creation, design similar structural manners and embodiments to this technical solution without creative efforts, they shall fall within the protection scope of this patent. In addition, the term "comprising" does not exclude other elements or steps, and the term "a" before an element does not exclude including "a plurality of" such elements. The plurality of elements stated in the product claims can also be implemented by one element through software or hardware. The terms such as "first" and "second" are used to indicate names and do not indicate any specific order.

Claims

1. A PERC cell, characterized in that, It includes a silicon nitride film layer on the back of the battery, and also includes a silicon wafer and a back alumina film layer provided on the back of the silicon wafer, and the back alumina film layer is located between the silicon wafer and the back silicon nitride film layer; The battery also includes a front silicon nitride film layer provided on the front of the silicon wafer; The silicon nitride film layer on the back of the battery includes a five-layer structure, and the thicknesses of the five-layer structure are α1, α2, α3, α4, and α5 from the inside to the outside in sequence, and the thicknesses of α1 to α5 are α5≥α3≥α1≥α4≥α2 or α5≥α3≥α1≥α2≥α4; The refractive indexes of the back five-layer silicon nitride film layer are β1, β2, β3, β4, and β5 from the inside to the outside in sequence, and the refractive indexes of β1 to β5 are β1≥β3>β4>β2=β5; The thickness value ranges of the back five-layer silicon nitride film layer are 25nm±5nm, 10nm±5nm, 30nm±5nm, 20nm±5nm, and 45nm±5nm from the inside to the outside in sequence, and the refractive index value ranges of the back five-layer silicon nitride film layer are 2.6±0.2, 2.1±0.1, 2.5±0.1, 2.3±0.05, and 2.1±0.1 from the inside to the outside in sequence.

2. A PERC cell according to claim 1, characterized in that, The thicknesses of the back five-layer silicon nitride film layer are 25nm, 10nm, 30nm, 20nm, and 45nm from the inside to the outside in sequence, and the refractive indexes of the back five-layer silicon nitride film layer are 2.6, 2.1, 2.5, 2.3, and 2.1 from the inside to the outside in sequence.

3. A method for preparing a PERC cell according to any one of claims 1-2, characterized in that, It includes the following steps: Step Ⅰ: Through operations such as texturing, diffusion, primary annealing, etching, and secondary annealing, a pre-treated silicon wafer is obtained; Step Ⅱ: Deposit a back alumina film layer on the back of the pre-treated silicon wafer; Step Ⅲ: Deposit a front silicon nitride film layer on the front of the pre-treated silicon wafer; Step Ⅳ: Deposit a back silicon nitride film layer on the back alumina film layer, and the process temperature is 470℃±20℃ when depositing the back silicon nitride film layer; Step Ⅴ: Use a laser to grove the back, screen-print to form a back electrode, a back electric field, and a positive electrode; sinter, and the preparation is completed.

4. The manufacturing method of a PERC cell according to claim 3, characterized in that, In step 4, the plasma-enhanced chemical vapor deposition is used for the back silicon nitride film layer.

5. The manufacturing method of a PERC cell according to claim 4, characterized in that, The plasma-enhanced chemical vapor deposition is a tube-type plasma-enhanced chemical vapor deposition.

6. The preparation method of a PERC cell according to claim 3, characterized in that, In step 2, the plasma-enhanced chemical vapor deposition is used for the front silicon nitride film layer.

7. The manufacturing method of a PERC cell according to claim 3, characterized in that In step 3, the back alumina film layer is formed by atomic layer deposition.

8. The preparation method of a PERC cell according to claim 7, characterized in that, In step 3, the back silicon nitride film layer uses gases such as NH3 and SiH4 as raw materials, and the refractive indexes of each layer of the five-layer film are controlled by adjusting the flow ratio of NH3 and SiH4 during production.

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