(1) Texturing

Raw material silicon wafer in the cutting process will form about 10 ~ 15 μm thick damage layer on the surface, this layer because of the state of the silicon wafer matrix has been different, basically has been peeled off to the matrix, will seriously affect the performance of solar cells, so to remove this damage layer, technical transformation project texturing process in the integrated texturing production line carried out, the main process includes pre-cleaning, texturing, post-cleaning, pickling 4 processes, each process after pure water cleaning, and finally drying.

Pre-cleaning process. Before the texturing of this project, monocrystalline silicon wafers first need to use potassium hydroxide and hydrogen peroxide solution to remove the damage layer and impurities on the surface of the silicon wafer, the initial concentration of lye is KOH: H2O2: water = 9: 24: 370, the temperature of the bath is about 70 °C, in the production process, hydrogen peroxide is both a strong oxidant and can be used as a reducing agent, organic matter, non-metal and most metal impurities in silicon oxide sheets. Hydrogen peroxide gradually decomposes under alkaline conditions, and potassium hydroxide is also consumed by the reaction. The equipment automatically replenishes the tank according to the concentration of the bath, and the replenishment rate of each tank is 0.26L/min, and the ratio is KOH:H₂O₂:water=0.45:5.5:5. Each production line is equipped with a total of 2 alkali washing tanks, the silicon wafer residence time is about 3min, the tank liquid is replaced once a day, and discharged to the wastewater treatment facility for treatment.

Pure water cleaning process 1. The pre-cleaned silicon wafer is then cleaned with pure water in the sink, the water volume of the cleaning tank is 360L, the cleaning water is automatically replenished, the replenishment acceleration rate is 15.88L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 15.88L/min.

Texturing process. After cleaning, the monocrystalline silicon wafer enters the alkali texturing section, and alkali texturing is carried out with an aqueous solution of potassium hydroxide and texturing additives. The initial concentration of lye is potassium hydroxide: additive: water = 12: 3: 390, during the production process, the equipment automatically replenishes the tank according to the concentration of the bath, the single tank replenishment rate is 1.649L/min, and the recharge ratio is KOH: additive: water = 9: 5.2: 34. Each production line is set up with 3 alkali texturing tanks, each batch of silicon wafer texturing takes about 9min, the texturing liquid is replaced once a day, the replacement amount of each tank is about 398L, and the wastewater is discharged to the wastewater treatment facility for treatment.

Pure water cleaning process 2. The silicon wafer after texturing needs to be cleaned with pure water, the water volume of the cleaning tank is 360L, the cleaning water is automatically replenished, the replenishment acceleration rate is 15.88L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 15.88L/min.

Post-cleaning process. The cleaned monocrystalline silicon wafer then enters the post-cleaning tank to clean the metal ions on the surface. After cleaning using hydrochloric acid aqueous solution, hydrochloric acid aqueous solution initial configuration concentration HCl: water = 1: 360, bath temperature is about 30 °C, during the production process, the equipment automatically replenishes acid according to the tank concentration, the single tank replenishment rate is 0.022L/min, the ratio is HCl: water = 0: 4.42.5, each batch of silicon wafer cleaning takes about 100s, the post-cleaning tank is replaced once a day, each replacement amount is 361L, each production line is set up with 1 tank, and the wastewater is discharged to the wastewater treatment facility for treatment.

Pure water cleaning process 3. After the post-cleaning treatment, the silicon wafer needs to be cleaned with pure water, the water volume of the cleaning tank is 360L, the cleaning water is automatically replenished, the replenishment acceleration rate is 15.88L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 15.88L/min.

Pickling process. The cleaned monocrystalline silicon wafer is then pickled in a pickling tank to remove SiO2 on the surface of the silicon wafer. Pickling adopts hydrofluoric acid, hydrochloric acid aqueous solution, the initial concentration of acid is HF: HCl: pure water = 30: 40: 120, the temperature of the bath is about 30 °C, during the production process, the equipment automatically replenishes the acid solution according to the concentration of the tank, the recharge rate is 0.127L/min, and the ratio is HF: HCl = 5.38: 4.04. Each batch of silicon wafers takes about 100s to clean, the pickling solution is replaced once a day, the amount of each replacement is 190L, each production line is set up with 1 pickling tank, and the wastewater is discharged to the wastewater treatment facility for treatment.

Pure water cleaning process 4. The silicon wafer after texturing needs to be cleaned with pure water, the water volume of the cleaning tank is 360L, the cleaning water is automatically replenished, the replenishment acceleration rate is 15.88L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 15.88L/min.

Hot water cleaning process. After the pure water cleaning, the silicon wafer is cleaned again with hot water, the water temperature is about 60 °C, each batch cleaning takes about 100s, the cleaning water is automatically replenished, and the overflow discharge waste is treated by the wastewater treatment facility, and the discharge rate per tank is about 15.88L/min.

Drying. After pure water cleaning, the silicon wafer is transferred to the dryer, and the silicon wafer is dried by hot air up and down, and the dryer adopts electric heating.

(2) Phosphorus diffusion

Put the pilened silicon wafer into the quartz tube of the diffusion machine, heat it up in the quartz container, and use nitrogen to carry oxygen and phosphorus into the tube under high temperature conditions to participate in the reaction; The phosphorus source compound is decomposed at high temperature, and the silicon wafer is in a quartz tube filled with the atmosphere of phosphorus atoms, and the phosphorus atoms enter the surface layer of the silicon wafer from all around, and penetrate and diffuse into the silicon wafer through the voids of the silicon atoms.

The project uses a liquid source phosphorus oxychloride to diffuse silicon wafers in a diffusion furnace. The diffusion furnace is a completely airtight device with electric heating. The specific process is: first pass a large flow of N₂ to drive away the air in the diffusion furnace quartz tube, and heat the diffusion furnace, wait for the furnace temperature to rise to 920 °C and constant, put the wafer into the quartz boat, send it to the furnace mouth for preheating for 5 minutes, and then push into the constant temperature zone for diffusion.

After the reaction is completed, the phosphorus atoms diffuse into the silicon wafer by re-deposition (PN junction). During the reaction, Si and O₂ were excessive, POCl₃ was completely rereacted, and the Cl₂ generated in the reaction was sent to the acid mist purification tower through the pipeline for treatment.

(3) Laser SE.

SE solar cells are selective emiters. The main characteristics of SE solar cells are high concentration doping of phosphorus in the metallized area and low concentration doping of phosphorus in the light area. The dense diffusion zone of the metallization area is deep, and impurities such as metals are not easy to enter the depletion zone to form a deep energy level during the sintering process, with small reverse leakage and high parallel resistance; The doping concentration in the illumination area is low, the short-wave response is good, and the short-circuit current is high; The front field of transverse diffusion high and low junctions has obvious effects, which is conducive to photogenerated carrier collection and other advantages. The efficiency of SE solar cells is higher than that of conventional solar cells, so some domestic companies study the use of different technologies to make SE solar cells.

(4) Wet etching.

Due to the diffusion process, a P-shaped layer is formed on the front and back of the silicon wafer, and a phosphosilicate glass is formed on the surface. After diffusion and knotification, it is necessary to remove the P-shaped layer on the back and edge of the silicon wafer, and corrode the silicon wafer again to remove the surface phosphorus silica glass formed by the silicon wafer during the diffusion process. In the corrosion process of the oxide layer, a small part of nitric acid and hydrofluoric acid react with silicon to form a complex of silicon.

Acid etching bath. The monocrystalline silicon wafer from the laser SE is then etched by acid, acid etching using hydrofluoric acid aqueous solution, the initial concentration of acid is HF: H₂O=35:400, the temperature of the bath is about 30 °C, during the production process, the equipment automatically replenishes the acid according to the concentration of the tank, the replenishment rate is 0.66L/min, each batch of silicon wafer cleaning takes about 80s, the pickling solution is replaced every two months, the amount of each replacement is 435L, and the wastewater is discharged to the wastewater treatment facility for treatment.

Pure water cleaning tank 1. The wet etched silicon wafer is then cleaned with pure water in the sink, the water volume of the cleaning tank is 150L, the cleaning water is automatically replenished, the replenishment acceleration rate is 8.82L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 8.82L/min per tank.

Alkali tank. Acid etched silicon wafer with potassium hydroxide pure aqueous solution for neutralization and cleaning, the initial concentration of lye is potassium hydroxide: water = 11: 145, the temperature of the bath is about 31 °C, during the production process, the equipment automatically replenishes the lye according to the concentration of the tank, the single tank replenishment rate is 0.17L/min, each production line is set up with a total of 1 alkali washing tank, each tank residence time is about 30s, the tank is replaced every two months, each tank capacity is 156L, and the wastewater is discharged to the wastewater treatment facility for treatment.

Pure water cleaning tank 1. The pre-cleaned silicon wafers are then cleaned with pure water in the sink, the water volume of the cleaning tank is 150L, the cleaning water is automatically replenished, the replenishment acceleration rate is 8.82L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 8.82L/min per tank.

Pickling tank. After washing, pickling is carried out, and pickling is carried out with an aqueous solution of HF. Each production line is set up with 1 pickling tank, each batch of silicon wafer texturing time is about 50s, pickling using hydrofluoric acid aqueous solution, hydrofluoric acid, water initial configuration concentration of HF: water = 100: 300, tank temperature is normal temperature, during the production process, the equipment automatically replenishes acid according to the concentration of the tank, the refill rate is 0.05L/min, the pickling solution is replaced once a day, the replacement amount of each tank is about 400L, and the wastewater is discharged to the wastewater treatment facility for treatment.

Pure water cleaning tank 1. The pre-cleaned silicon wafers are then cleaned with pure water in the sink, the water volume of the cleaning tank is 150L, the cleaning water is automatically replenished, the replenishment acceleration rate is 8.82L/min, and the overflow is discharged to the wastewater treatment facility for treatment. Each production line is equipped with a cleaning tank with a discharge rate of about 8.82L/min per tank.

Drying. After pure water cleaning, the silicon wafer is transferred to the dryer, and the silicon wafer is dried by hot air up and down, and the dryer adopts electric heating.

(5) Thermal oxidation.

After laser SE treatment, in order to improve the toughness of the cell, improve the crystalline silicon structure and its electrical properties, the cells are annealed in an annealing furnace with oxygen. The annealing furnace heating method is electric heating.

(6) Back passivation.

The backpassivation process of this product is the same as that of large-size silicon wafers, but the parameters are different

(7) Prepare deposited silicon nitride film on the front.

The front-side deposited silicon nitride film is the same as the backside deposited silicon nitride film, and will not be repeated here. The company has 9 sets of PECVD back passivation coating machines, and the main production process is the same as the existing production process.

A small amount of H₂ and excess SiH4 and NH₃ reflected in the formation enter the stainless steel silane combustion column for processing.

(8) Laser grooving.

Since AlOx is a dense membrane, it is not possible to subsequently form a backelectric field and draw current out of the back of the silicon wafer. Therefore, it is necessary to use laser slotting to draw a point or line on the back of the silicon wafer, and remove the local AlOx film to form an aluminum back field in this local area and draw the current out to form the positive electrode of the solar cell. The dust generated during the silicon wafer laser process is treated by a high-efficiency air filtration system, and the purified air enters the closed and clean workshop of the project through the internal circulation, so there is no exhaust gas emission.

(9) Screen printing, sintering.

Screen printing is to print the silver paste on the back and front of the silicon wafer with a screen printing machine, and then pass through the sintering furnace, and the silver paste printed on the back and front of the solar cell penetrates into the inside of the silicon wafer at a certain temperature to enhance the conductivity and form a silver electrode on the front and back surfaces of N-type double-sided solar cells. The purpose of screen printing is to make upper and lower electrodes that collect the charge generated by the photovolt effect. The silver paste used in screen printing is a paste printing paste made of ultra-fine and high-purity silver powder as the main metal, with a certain amount of organic binder and resin as auxiliary agents. The purpose of sintering is mainly to form a sintered gold and ohmic contact and remove the back knot. The sintering process is actually a high-temperature diffusion process, which is a process of doping silicon and needs to be heated above the aluminum-silicon eutectic point (577 °C). After alloying, with the decrease of temperature, the silicon in the liquid phase will be resolidified to form a crystalline layer containing a small amount of aluminum, which compensates for the donor impurities in the P layer, so as to obtain the N layer with aluminum as the main impurity, achieving the purpose of eliminating the back junction.

(10) Test sorting.

After the solar cell is naturally cooled to room temperature, it is sent to the solar cell electrical performance tester for testing, and after testing, the solar cell will be classified according to its conversion efficiency, different colors and poor appearance. In the introduction to the principle of solar cells, it is mentioned that the solar cell is a small number of carriers working principle, therefore, the minority carrier life directly affects the conversion efficiency of the cell, and the photogenerated carriers generated by the sunlight absorption surface can not be completely converted into electrical energy, because there are many single crystal defects and impurity atoms in the body and surface of silicon materials, and these defects and impurities will make the photogenerated carriers in the body and the surface be recomposed, so that the number of carriers that can produce photovoltaic effects is reduced, reducing the conversion efficiency of electrons. Sorting is to classify the silicon wafer according to the conversion efficiency of the electron, and after grading, the silicon wafer is divided to obtain the finished solar cell.