A backside silicon nitride opening method for a PERC cell

Abstract

The present application relates to the field of solar cell production, a back surface silicon nitride hole opening method for a PERC cell, which adopts a printing hole opening paste + hole opening paste drying method to perform hole opening after back surface passivation and front surface passivation. The hole opening paste contains 10-30% glass powder, 50-70% aluminum powder, 10-20% organic solvent and 5-10% additive in terms of mass percentage, the hole opening paste is printed to the position of the silicon wafer requiring hole opening through printing, and then the hole opening paste on the silicon wafer is dried, and the hole opening paste drying temperature is 200-500 DEG C. The chemical hole opening is realized through the hole opening paste, replacing the current laser hole opening, and the damage of the laser to the silicon wafer at the hole opening position is avoided.

Description

Technical Field

[0001] The present invention relates to the field of solar cell production, in particular to the field of post-passivation opening of silicon wafer back surfaces. Background Art

[0002] Compared to traditional cells, the core of PERC cells lies in back-side passivation and back contact. This comprehensive back-side passivation improves the cell's conversion efficiency. As key processes in PERC cells, the porosity, hole pattern, and slurry filling are key factors limiting PERC cell efficiency. Laser perforation uses a laser to drill holes or grooves on the back of a silicon wafer, partially penetrating the passivation film to expose the silicon substrate. The back electric field then contacts the silicon substrate through the holes or grooves in the film.

[0003] In the existing technology, laser drilling is generally used when drilling holes in the back passivation of PERC cells. During the drilling process, the high-energy laser beam instantly breaks through the passivation film and instantly melts the local silicon wafer, which undoubtedly increases the damage to the cell. At the same time, the fluidity of the slurry on the uneven melted silicon wafer deteriorates, and after drying, there is a failed contact with the silicon wafer, which reduces the probability of electron transmission and reduces the conversion efficiency of the cell. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: how to achieve chemical drilling through drilling slurry to replace the current laser drilling, reduce the damage of the laser to the silicon wafer at the drilling position, and at the same time, the slurry at the drilling position can more easily fill the drilling position completely, thereby improving the contact performance and improving the conversion efficiency of the battery.

[0005] The technical solution adopted by the present invention is: a method for opening holes in the back side silicon nitride of a PERC battery, which is carried out according to the process steps of texturing, diffusion, SE+front oxidation, PSG removal+back polishing, thermal oxidation, back passivation, front passivation, printing opening slurry+opening slurry drying, back silver printing+back silver drying, back aluminum printing+back aluminum drying, front silver printing, sintering, and electrical injection, wherein the opening slurry contains 10-30% glass powder, 50-70% aluminum powder, 10-20% organic solvent, and 5-10% additives by mass percentage. The opening slurry is printed on the position where the hole needs to be opened on the silicon wafer, and then the opening slurry on the silicon wafer is dried. The drying temperature of the opening slurry is 200-500°C. After the back aluminum printing+back aluminum drying process is completed, it is baked and fused. The baking and fusion temperature is 700-800°C, and the baking and fusion is carried out for 3-5 minutes.

[0006] In the open-pore slurry, the glass powder contains, by mass, 25-40 parts of V2O5, 20-30 parts of ZnO, 15-25 parts of P2O5, 5-15 parts of Sb2O3, 8-12 parts of Bi2O3, and 0.1-3 parts of Na2O; in the open-pore slurry, the median diameter D50 of the aluminum powder is 1-3 μm; in the open-pore slurry, the organic solvent is one or a mixture of alcohol ester dodecanol, terpineol, and diethylene glycol butyl ether acetate; in the open-pore slurry, the additives contain, by mass, 3-5% of ethyl cellulose, 4-8% of styrene acrylic resin, 10-30% of ethyl cellulose, 10-30% of alcohol ester dodecanol, and 10-30% of butyl carbitol.

[0007] The drying temperature for open-pore slurry drying is 200-500℃, the drying temperature for back silver drying is 300-500℃, and the drying temperature for back aluminum drying is 300-500℃.

[0008] After the thermal oxidation process is completed, the silicon wafer is immersed in the mixed slurry solution, dried at 300-500°C, and then subjected to the back side passivation process.

[0009] The mixed slurry solution contains, by mass percentage, 6-7% aluminum powder, 1-2% glass powder, 5-8% organic solvent, and 80-90% additives, wherein the glass powder contains, by mass, 25-40 parts of V2O5, 20-30 parts of ZnO, 15-25 parts of P2O5, 5-15 parts of Sb2O3, 8-12 parts of Bi2O3, and 0.1-3 parts of Na2O; the organic solvent is a mixture of one or more of alcohol ester dodecanol, terpineol, and diethylene glycol butyl ether acetate; and the additives contain, by mass, 3-5% of ethyl cellulose, 4-8% of styrene acrylic resin, 10-30% of ethyl cellulose, 10-30% of alcohol ester dodecanol, and 10-30% of butyl carbitol.

[0010] The beneficial effects of the present invention are as follows: the present invention uses glass powder, aluminum powder, organic solvent and additives to mix into an open hole slurry, and uses the screen printing method to evenly print a thin layer of open hole slurry at the position where the silicon nitride on the back of the PERC battery needs to be opened. During the drying process at 200-500 ° C, the glass powder is quickly used to melt the silicon nitride, and then the aluminum powder and organic matter enter the open hole position and form a good contact with the silicon wafer. At the same time, the high temperature process evaporates the organic matter and additives, and finally the slurry at the open hole position is in close contact with the silicon wafer. Then, by printing the back silver and drying it, printing the back aluminum and drying it, and sintering it, the aluminum powder at the open hole position, the printed aluminum paste and the silicon wafer form a good ohmic contact. By using two aluminum paste filling and printing, the open hole position is filled more fully, the contact performance is better, and the conversion efficiency is 0.05-0.1% higher. DETAILED DESCRIPTION

[0011] A method for opening holes in the back side silicon nitride of a PERC cell is carried out according to the process steps of texturing, diffusion, SE+front oxidation, PSG removal+back side polishing, thermal oxidation, back side passivation, front side passivation, printing opening slurry+opening slurry drying, back side silver printing+back side silver drying, back side aluminum printing+back side aluminum drying, front side silver printing, sintering, and electrical injection.

[0012] Velveting

[0013] 0.5g of a nucleating agent, 1g of anionic surfactant, 0.5g of a dispersant, and 0.5g of a chelating agent were added to deionized water to prepare a 1000g mixed solution and stirred evenly to obtain a crystalline silicon texturing additive. The nucleating agent consisted of 0.1g of β-cyclodextrin and 0.4g of lignin sulfonate; the anionic surfactant consisted of 0.6g of alkylbenzene sulfonate and 0.4g of alkyl sulfonate; the dispersant consisted of 0.5g of polyacrylamide; and the chelating agent consisted of 0.25g of sodium formate and 0.25g of sodium acetate.

[0014] The obtained texturing additive is mixed evenly according to a formula of 3% strong base, 2% texturing additive, and the remainder deionized water to obtain a crystalline silicon texturing agent. The strong base is potassium hydroxide.

[0015] The cleaned single crystal silicon wafer was immersed in a texturing solution for reactive etching. The texturing solution temperature was 80°C and the texturing time was 420s to obtain a P-type silicon wafer with an inverted pyramid texture structure.

[0016] diffusion

[0017] The cleaned and textured P-type silicon wafer is placed face up in a quartz boat and placed together with the quartz boat into a tubular diffusion furnace. The vacuum is evacuated to a pressure of 100 Pa, and then the temperature is raised to 750°C. Oxygen is introduced at a flow rate of 500 sccm for pre-oxidation. The pressure in the furnace is controlled at 100 mbar and the pre-oxidation time is 3 minutes to form a silicon dioxide layer.

[0018] After the pre-oxidation is completed, the temperature is first raised to 770°C and then constant temperature is used for primary deposition, and oxygen with a flow rate of 400sccm and nitrogen with a phosphorus source of 500sccm are introduced to deposit the phosphorus source on the surface of the silicon wafer. The temperature is then continued to be raised to 790°C, and secondary deposition is carried out from the start of the temperature increase, and oxygen with a flow rate of 600sccm and nitrogen with a phosphorus source of 700sccm are introduced. The pressure in the furnace is controlled to 100mbar during the primary and secondary depositions, and nitrogen with a flow rate of 400sccm is additionally introduced. The time for the primary and secondary depositions is 3min;

[0019] Stop the introduction of oxygen and phosphorus-carrying nitrogen, continue to heat up to 860°C and maintain constant temperature, perform passive propulsion treatment during the heating and constant temperature stages, introduce nitrogen with a flow rate of 1500sccm to maintain the pressure at 100mbar, and the total time of the passive propulsion treatment is 15min; cool to 770°C, introduce oxygen with a flow rate of 400sccm and phosphorus-carrying nitrogen with a flow rate of 400sccm again, and additionally introduce nitrogen with a flow rate of 400sccm. The pressure in the furnace is controlled at 100mbar, and three depositions are performed, with a deposition time of 2min;

[0020] Stop introducing nitrogen gas carrying the phosphorus source, and introduce oxygen gas with a flow rate of 500sccm for secondary oxidation. The temperature of the secondary oxidation is 770°C. During the secondary oxidation, nitrogen gas with a flow rate of 400sccm is also introduced. The pressure in the furnace is controlled to be 100mbar, and the time of the secondary oxidation is 8min. Then, oxygen gas with a flow rate of 500sccm and nitrogen gas carrying the phosphorus source with a flow rate of 800sccm are introduced simultaneously to carry out four depositions. The deposition temperature is 770°C. Nitrogen gas with a flow rate of 400sccm is also introduced. The pressure in the furnace is controlled to be 100mbar, and the deposition time is 5min. Then, stop introducing oxygen and nitrogen gas carrying the phosphorus source, introduce nitrogen gas for pressurization after cooling, take out the quartz boat, unload the silicon wafer, and obtain the silicon wafer after diffusion treatment.

[0021] SE+pre-oxidation

[0022] The SE laser doping pattern is made by: using laser irradiation with a wavelength of 532nm, designing the SE laser doping pattern according to the battery front electrode pattern, selecting Nd:YAG laser with a wavelength of 532nm, installing a shaping mirror on the laser, and making the spot a square spot, which is beneficial to reduce the damage of the laser to the silicon wafer. The scanning speed is 10-100000mm / s, the spot width is 40-150um, the laser frequency is 10-5000KHz, the pulse energy is 10-120uJ / pulse, and the power per unit area is 2-80J / cm 2 .

[0023] A 3-5nm thick silicon oxide film is prepared on the front and back sides of the silicon wafer.

[0024] Remove PSG + back polishing

[0025] A water film spraying device is installed in the feeding section. After the water film thinning roller carries the silicon wafer through the water film spraying device, its upper surface is evenly covered with a layer of water film. The feeding water film spraying device sprays the upper surface of the silicon wafer for 10 to 12 seconds. After the water film covers the upper surface of the silicon wafer, a protective layer can be formed. In this way, when the next process is transferred to the next step, the splashing of the chemical solution in the PSG tank will not corrode the upper surface of the silicon wafer, thereby preventing the upper surface of the silicon wafer from becoming hydrophobic.

[0026] The silicon wafer enters the single-sided PSG tank and the upper surface does not contact the liquid in the tank. At the same time, the pump body circulates and stirs the tank body with the prepared liquid concentration, and sends the liquid into the single-sided PSG tank. After the temperature and liquid level conditions in the single-sided PSG tank are met, the PLC issues a working instruction, and the silicon wafer reacts with the liquid on its lower surface through the roller. The immersion time of the liquid on the lower surface of the silicon wafer is 35 to 37 seconds and the temperature inside the liquid is at room temperature. The liquid in the single-sided etching is tetramethylammonium hydroxide, and tetramethylammonium hydroxide The concentration of ammonium is 8%. Finally, tetramethylammonium hydroxide must completely remove the PSG layer and the surrounding area on the lower surface of the silicon wafer, and there must be no water droplets adhering to the lower surface after immersion. At the same time, the PSG layer on the upper surface of the silicon wafer cannot be damaged, and the upper surface can obviously absorb water after immersion. Since tetramethylammonium hydroxide is an alkaline corrosive, it is irritating and corrosive, and the smoke can cause respiratory irritation and conjunctivitis. Therefore, during the single-sided etching operation, an exhaust device must be used for ventilation, and the ventilation rate is 800Nm 3 / h, to prevent the volatile smoke of tetramethylammonium hydroxide from causing burns to the human body.

[0027] Water washing: After the silicon wafer is etched by the chemical solution, it is carried out of the single-sided PSG tank by the water film thinning roller and passed through the water film spray device again for all-round spray washing. The water flow spraying time for the silicon wafer is 25 to 30 seconds to prevent the tetramethylammonium hydroxide solution remaining on the surface of the silicon wafer from polluting the environment.

[0028] Blow dry; after the silicon wafer is rinsed, a strong wind is blown through the air duct for about 14 seconds to further clean the tetramethylammonium hydroxide solution remaining on the surface of the silicon wafer to prevent the operator from directly contacting the silicon wafer and causing skin and eye burns.

[0029] Discharging: After the silicon wafer is dried, it is removed to complete the single-sided PSG layer removal process.

[0030] Use the same water film protection as before.

[0031] Etching: The silicon wafer is completely immersed in a tank containing a chemical solution and a temperature of 65°C. After the silicon wafer is completely immersed in the tank, it is not paused, and then the silicon wafer is directly clamped out. The etching must ensure that the edge insulation etching of the silicon wafer is completed, and the edge insulation resistance of the silicon wafer with an edge spacing of 1 cm is 4 to 30KΩ. At the same time, the unetched edge ratio of the silicon wafer is ≤1‰. Tetramethylammonium hydroxide further corrodes the surface of the silicon wafer and the surface roughness reaches less than 50nm.

[0032] Wash with water; take out the silicon wafer and completely immerse it in the water tank and then take it out, and the water in the water tank is in an overflow state, which can ensure that the tetramethylammonium hydroxide on the surface of the silicon wafer is rinsed cleanly by the overflowing water at one time.

[0033] HF pickling: After the silicon wafer is removed from the water tank, it is immersed in HF at a temperature of 25-35°C without stopping. After the silicon wafer is completely immersed in HF, it is directly clamped out. At the same time, the HF concentration is 49%. According to the reaction equation SiO2+4HF→SiF4+2H2O, HF can further remove the phosphosilicate glass on the surface of the silicon wafer, making the wafer hydrophobic. At the same time, the smoothness of the entire surface of the silicon wafer is further improved, which is beneficial to reduce the scattering of sunlight on the surface of the silicon wafer, so that the photoelectric conversion rate of the silicon wafer reaches about 14%. Because HF is highly irritating and corrosive to the skin and can produce acid mist, an exhaust device must be used for ventilation during the HF pickling operation, and the ventilation rate is 1000Nm3 / h to prevent the acid mist emitted by HF from causing bronchitis and hemorrhagic pulmonary edema to the human body.

[0034] Wash with water; remove the silicon wafer and immerse it completely in the water tank again and take it out. The water in the water tank is in an overflowing state, which can rinse the HF solution on the surface of the silicon wafer at one time.

[0035] Unloading: After the silicon wafer is cleaned, it is taken out and the polishing process is completed.

[0036] Thermal oxidation

[0037] The current thermal oxidation process primarily involves introducing a certain amount of oxygen at high temperatures to create a thin layer of silicon dioxide on the surface of the silicon wafer. This effectively reduces dangling bonds at the silicon-silicon dioxide interface, lowering the interface state density and improving the passivation effect of the oxide layer. Simultaneously, high temperatures activate phosphorus atoms accumulated on the silicon wafer surface, reducing surface concentration and the dead layer. Thermal oxidation effectively repairs lattice defects and dangling bonds on the silicon wafer surface, reducing recombination centers.

[0038] In the low-pressure thermal oxidation furnace, the temperature is 650°, nitrogen is introduced at a flow rate of 10,000 sccm, oxygen is introduced at a flow rate of 2,800 sccm, the reaction time is 250 seconds, and the pressure in the second furnace is 1,060 Pa.

[0039] After the thermal oxidation process is completed, the silicon wafer is immersed in a mixed slurry solution, dried at 400°C, and then subjected to a backside passivation process. The mixed slurry solution contains, by weight, 7% aluminum powder, 2% glass powder, 6% organic solvent, and 85% additives. The glass powder contains, by weight, 30 parts V2O5, 20 parts ZnO, 21 parts P2O5, 10 parts Sb2O3, 9 parts Bi2O3, and 2 parts Na2O. The organic solvent is alcohol ester dodecahydrate. The additives include, by weight, 5% ethyl cellulose, 5% styrene acrylic resin, 30% ethyl cellulose, 30% alcohol ester dodecahydrate, and 30% butyl carbitol.

[0040] The back side passivation and the front side passivation are performed by forming a silicon nitride passivation layer using existing technology.

[0041] Printing open-pore slurry + drying open-pore slurry

[0042] The opening slurry contains 30% glass powder, 60% aluminum powder, 15% organic solvent and 5% additives by mass. The opening slurry is printed on the position where the silicon wafer needs to be opened, and then the opening slurry on the silicon wafer is dried. The drying temperature of the opening slurry is 300°C. After the back aluminum printing + back aluminum drying process is completed, it is baked and fused. The baking and fusion temperature is 800°C and the baking and fusion is carried out for 3 minutes.

[0043] In the open-pore slurry, the glass powder contains, by mass, 25-40 parts of V2O5, 20-30 parts of ZnO, 15-25 parts of P2O5, 5-15 parts of Sb2O3, 8-12 parts of Bi2O3, and 0.1-3 parts of Na2O; in the open-pore slurry, the median diameter D50 of the aluminum powder is 1-3 μm; in the open-pore slurry, the organic solvent is one or a mixture of alcohol ester dodecanol, terpineol, and diethylene glycol butyl ether acetate; in the open-pore slurry, the additives contain, by mass, 3-5% of ethyl cellulose, 4-8% of styrene acrylic resin, 10-30% of ethyl cellulose, 10-30% of alcohol ester dodecanol, and 10-30% of butyl carbitol.

[0044] Back-silver printing followed by back-silver drying, or back-aluminum printing followed by back-aluminum drying. The drying temperature for back-silver drying is 400°C, and for back-aluminum drying is 400°C. Both back-silver and back-aluminum printing processes utilize existing slurries. After the back-aluminum printing and back-aluminum drying processes are complete, a baking process is performed at a temperature of 700-800°C for 3-5 minutes.

[0045] Front silver printing, sintering and electrical injection all use existing technologies.

[0046] The key point of the present invention is to use a pore-forming paste containing glass powder and aluminum powder, print it on the location where the PERC cell needs to be opened, dry it at a temperature of 200-500°C, so that the glass powder can fully melt the silicon nitride on the PERC surface. At the same time, the aluminum powder, lubricated by organic matter, can completely fill the corroded silicon nitride holes. Then, the organic matter is fully evaporated at high temperature, so that the aluminum powder forms a good contact with the silicon wafer. After printing and drying the back silver, back aluminum, and front silver, the aluminum paste and silver paste are eutectic with the silicon wafer to form an alloy, forming a good ohmic contact.

Claims

1. A method for opening a hole in the back side silicon nitride layer of a PERC cell, characterized by: According to the process steps of texturing, diffusion, SE+front oxidation, PSG removal+back polishing, thermal oxidation, back passivation, front passivation, printing opening paste+opening paste drying, back silver printing+back silver drying, back aluminum printing+back aluminum drying, front silver printing, sintering, and electric injection, the opening paste contains 10-30% glass powder, 50-70% aluminum powder, 10-20% organic solvent, and 5-10% additives by mass percentage. The opening paste is printed on the position where the silicon wafer needs to open the hole, and then the opening paste on the silicon wafer is subjected to The pore slurry is dried at a temperature of 200-500°C. After the back aluminum printing and back aluminum drying processes are completed, baking and fusion are carried out at a temperature of 700-800°C for 3-5 minutes. After the thermal oxidation process is completed, the silicon wafer is immersed in the mixed slurry solution and dried at 300-500°C, and then the back passivation process is carried out. The mixed slurry solution contains 6-7% aluminum powder, 1-2% glass powder, 5-8% organic solvent, and 80-90% additives by mass percentage, wherein the glass powder contains V2O5 by mass. 25-40 parts, ZnO 20-30 parts, P2O5 15-25 parts, Sb2O3 5-15 parts, Bi2O3 8-12 parts, Na2O 0.1-3 parts; the organic solvent is a mixture of one or more of alcohol ester dodecanol, terpineol, and diethylene glycol butyl ether acetate; the additives include 3-5% by mass of ethyl cellulose, 4-8% by mass of styrene acrylic resin, 10-30% by mass of ethyl cellulose, 10-30% by mass of alcohol ester dodecanol, and 10-30% by mass of butyl carbitol.

2. The method for forming a hole in the back side silicon nitride layer of a PERC cell according to claim 1, wherein: In the open-pore slurry, the glass powder contains, by mass, 25-40 parts of V2O5, 20-30 parts of ZnO, 15-25 parts of P2O5, 5-15 parts of Sb2O3, 8-12 parts of Bi2O3, and 0.1-3 parts of Na2O; in the open-pore slurry, the median diameter D50 of the aluminum powder is 1-3 μm; in the open-pore slurry, the organic solvent is one or a mixture of alcohol ester dodecanol, terpineol, and diethylene glycol butyl ether acetate; in the open-pore slurry, the additives contain, by mass, 3-5% of ethyl cellulose, 4-8% of styrene acrylic resin, 10-30% of ethyl cellulose, 10-30% of alcohol ester dodecanol, and 10-30% of butyl carbitol.

3. The method for opening a hole in the back side silicon nitride of a PERC cell according to claim 1, wherein: The drying temperature for open-pore slurry drying is 200-500℃, the drying temperature for back silver drying is 300-500℃, and the drying temperature for back aluminum drying is 300-500℃.