The preparation process of the TOPCon solar cells includes cleaning texture, BSG removal and back etching, oxide layer passivation contact preparation, front aluminum oxide deposition, front and back silicon nitride deposition, screen printing, sintering, and test sorting, about 12 steps about.
Among them, boron diffusion, SE laser re-doping, post-oxidation, passivation layer film deposition (CVD), etc., are the core process links.
This blog will go into detail one by one, please continue reading.
Texturing section (6 lines in total)
1- Purpose of pre-cleaning: To remove impurities (organic and metal impurities, etc.) adhering to the surface of the silicon wafer, using NaOH solution and H2O2 solution.
2- Alkali Texture Purpose: Carry out anisotropic corrosion of the crystal on the silicon surface by lye to form a pyramid with a size of 5um on the surface.
The pyramid texture has excellent light trapping and anti-reflection effects (10%). Alkali texturing uses NaOH solution and texturing additives.
3- Post-cleaning the silicon wafer after alkali texturing enters the post-cleaning tank to remove residual organic matter.
And ensure the cleanliness of the silicon wafer surface, thereby improving the battery conversion efficiency to a certain extent.
4-Pickling After the post-cleaning, dilute acid solution (3.15% HCl and 7.1% HF) should be used for high-purity cleaning.
The function of HCl is to neutralize the residual NaOH, and the function of HF is to remove the oxide layer on the surface of the silicon wafer.
Make the surface of the silicon wafer more hydrophobic and form a silicon complex H2SiF6.
The metal ions are detached from the surface of the silicon wafer through the complexation with the metal ions.
So that the metal ion content of the silicon wafer is reduced to prepare for the diffusion junction. Clean with pure water after pickling.
5- Slow pulling pre-dehydration purpose: to pre-dehydrate the surface of the crystal silicon wafer, usually as the last step in the pure water cleaning process.
The crystalline silicon wafers cleaned with pure water are transferred to the slow-pulling tank.
Silicon wafers are first submerged in pure water completely soaked, and then slowly lifted upwards by the manipulator and the hanging basket.
The surface tension can pull down the water film on the silicon wafers.
6- Drying: Transfer the crystalline silicon wafer to the drying tank, blow the hot air at 90°C to the upper and lower sides of the silicon wafer for drying, and use electric heating for drying.
Core Process
Boron diffusion
SE laser doping
Post-oxidation
Etching
1- Removing BSG: The BSG on the back side of the silicon wafer is removed by floating in water in a chain cleaner.
Back side is in contact with an acid solution. The main component of the acid solution is 24.5% HF.
2) Back etching: To improve the reflectivity of the back of the silicon wafer, the back of the silicon wafer is polished by an alkali and polishing agent.
The alkali polishing section (6 lines) includes pre-cleaning-water washing-alkali polishing*2, hydrogen peroxide cleaning (reserved), micro-texturing (reserved), pure water cleaning, post-cleaning, pure water cleaning, and acid.
Washing*2, pure water washing after pickling, slow pulling, and pre-dehydration, drying*5 and other modules.
3- Pre-cleaning The processed silicon wafers enter the cleaning tank to remove residual organic matter and ensure the cleanliness of the silicon wafer surface, thereby improving the battery conversion efficiency to a certain extent.
4) Alkali throwing The alkali throwing tank is equipped with pure water, and an appropriate amount of NaOH solution and polishing additives are added.
NaOH solution is about 1.6%, and the concentration of the polishing agent is 0.97%.
And then the back surface of the silicon wafer is polished. is 65°C. Wash with pure water after washing with alkali.
5- Add pure water to the post-cleaning and micro-velvet tank, and add an appropriate amount of NaOH solution and hydrogen peroxide.
NaOH solution is about 0.55%, hydrogen peroxide concentration is 0.25%.
According to the ratio to cleaning at room temperature. Clean with pure water after cleaning.
6) After pickling, dilute acid solution (0.9% HCl and 0.23% HF) should be used for high-purity cleaning.
Function of HCl is to neutralize the residual NaOH, and the function of HF is to remove the oxide layer on the surface of the silicon wafer. Make the wafer surface more hydrophobic.
The silicon complex H2SiF6 is formed, and the metal ions are detached from the surface of the silicon wafer through the complexation with the metal ions.
So that the metal ion content of the silicon wafer is reduced, preparing for the diffusion junction. Clean with pure water after pickling.
7- Drying: Transfer the slow-pulling and pre-dehydrated crystalline silicon wafers to the drying tank, blow 90°C hot air up and down the silicon wafers for drying, and use electric heating for drying.
POPAID Deposition In Situ Doping
First, the silicon wafer enters the loading chamber under the atmospheric environment, is transported into the 300° preheating chamber, and then enters the PO process chamber.
At this time, O2 is transported to the gas distribution block through the trachea and is activated and ionized by the RF radio frequency power supply.
And the ions are oxidized on the surface of the silicon wafer to form a tunnel oxide layer.
Then the silicon wafer passes through the transition and buffer cavity and is transferred into the paid cavity, and the paid source deposits a certain thickness of amorphous silicon on the back of the substrate.
At the same time, PH3 gas is fed during the deposition process, the gaseous phosphine enters the machine.
The phosphorus in the phosphine is excited into the state of phosphorus ions by 10kev and 0.5-2kev high-voltage radio frequency.
A DC high voltage is added between the ion source and the ground, so that the phosphorus ions obtain energy through the high-voltage electric field, and the width of the beam is 420mm.
Then the silicon wafer is transported under the beam, and the atoms of the paid source fly to the substrate to carry P ions or react with P ions to achieve in-situ phosphorus doping.
Annealing
The silicon wafer is placed in a reaction tube made of quartz glass, and the reaction tube is heated to a certain temperature by a resistance wire heating furnace.
Commonly used temperature is 900-1200°C, and it can be lowered to below 600°C under special conditions).
When oxygen passes through the reaction tube, a chemical reaction occurs on the surface of the silicon wafer: Si (solid-state) + O2 (gaseous state) → SiO2 (solid state).
The redistribution of impurities generated during the annealing process also acts as a gettering effect, and the adsorption and fixation of sodium and potassium ions by PSG are used to remove these harmful ions.
Analysis of pollution production links: The main pollution links in this process are residual oxygen and nitrogen in the thermal oxygen link.
BOE cleaning
BOE (5 lines) trough equipment is an integrated semi-closed equipment.
The silicon wafers are placed in the basket by the automatic equipment and are transferred in the solution of each tank in the equipment through the mechanical arm.
Among them, the chemical tank is continuously replenished with corresponding chemicals according to the concentration of the solution and is replaced as a whole regularly.
The replaced waste liquid is discharged into the wastewater system and finally enters the sewage treatment station for treatment.
1- Pickling: It needs to use a dilute acid solution (3.15% HCl and 7.1% HF) for high-purity cleaning.
Function of HCl is to use Cl- to complex metal ions, and the function of HF is to remove the oxide layer on the surface of the silicon wafer to make the surface of the silicon wafer more hydrophobic.
The silicon complex H2SiF6 is formed, and the metal ions are detached from the surface of the silicon wafer through the complexation with the metal ions.
To reduce the metal ion content of the silicon wafer, HF pickling for 150 to remove the BSG layer on the front and the PSG layer on the back side, followed by pure water cleaning after pickling.
2) Post-acid cleaning After post-cleaning, a dilute acid solution (14.7% HF) should be used for high-purity cleaning.
Function of HF is to remove the oxide layer on the surface of the silicon wafer to make the surface of the silicon wafer more hydrophobic.
The silicon complex H2SiF6 is formed, and the metal ion is detached from the surface of the silicon wafer through the complexation with the metal ion so that the metal ion content of the silicon wafer is reduced.
3) Drying Transfer the slowly pulled and pre-dehydrated crystalline silicon wafers to the drying tank, blow 90°C hot air up and down the silicon wafers for drying, and use electric heating for drying.
ALD
Use ALD equipment to plate a layer of Al2O3 on the surface of the silicon wafer to improve the passivation and gettering effect of the silicon wafer surface.
It mainly uses the reaction of gaseous Al(CH3)3 and water vapor (H2O) to generate Al(OH)3, which adheres to the surface of the silicon wafer and generates methane gas at the same time.
Front Coating
The basic principle is to use high-frequency photodischarge to generate plasma to exert influence on the film deposition process, promote the decomposition, combination, excitation, and ionization of gas molecules, and promote the generation of reactive groups.
Since the presence of NH3 is beneficial to the flow and diffusion of active groups, the growth rate of the film is improved, and the deposition temperature is greatly reduced.
Backside Coating
The PECVD back film equipment is closed negative pressure equipment, electric heating, and comes with an oil-free dry mechanical vacuum pump.
During production, nitrogen gas is first filled into the equipment, and the mechanical arm completes the loading of silicon wafers into the boat.
After the equipment reaches the external pressure, the inlet and outlet are opened, and the graphite boat automatically enters the equipment, and the inlet and outlet are closed.
Vacuumize and carry out various safety inspections. After confirming safety, silane, and ammonia gas are introduced to complete the silicon nitride oxide coating in the equipment.
After the coating is completed, the residual gas in the special gas pipeline and the equipment is discharged through nitrogen, and then the inlet and outlet are opened to discharge the material.
Until cooling, it enters into finishing and enters the follow-up process.
Metalization
1) During the printing process, the slurry is above the screen, and the scraper presses against the screen with a certain pressure, so that the screen deforms and contacts the surface of the silicon wafer.
2- Sintering is to sinter the main fine grid paste printed on the silicon wafer into a cell at high temperature.
So that the electrodes are embedded in the surface, forming a firm mechanical contact and good electrical connection, and finally forming an ohmic contact between the electrode and the silicon wafer itself.
3) After the sintering of the electric injection cell, use the method of direct electric injection carrier (reverse injection direct current) to change the charged state of the hydrogen in the silicon body.
So that the attenuated boron-oxygen complex can be well passivated.
It transforms into a stable regeneration ecology, and finally achieves the purpose of anti-light decay.
The final production step is the conventional test packaging. After passing the test, it can be packaged and shipped to you.
Future trend
N-type solar panels are divided into three types: TOPCon, HJT, and IBC, and their energy conversion efficiency is about 25.5%.
This P-type solar panel is about 2 points higher.
According to authoritative forecasts, by 2030, the market share of N-type will reach about 56%.
Although there are three types of N-type solar panels, only TOPCon solar cells and HJT are currently commercially popularized and applied.
Currently, TOPCon solar panel has a higher penetration rate.
1- High efficiency: The efficiency of the manufacturer with the highest mass production efficiency in TOPCon solar cells is as high as 24.8%, the current mainstream PERC is 23.2%, and TOPCon is 1.6% higher.
It is estimated that by the second half of 2023, TOPCon solar cells can reach 26.8%, PERC’s efficiency is around 23.5%, and the efficiency difference can reach 3.3%.
2- Low attenuation: TOPCon solar cells do not drop gears, while the current test efficiency of PERC solar panels drops by 1 to 2 gears after some time.
On the other hand, for the component side, there is no attenuation in the power generation of TOPCon solar cells, and the power attenuation of PERC is close to 8% in the first year.
3- Good long-wave response: PERC only responds well to short-wave, while TOPCon solar cells respond well to both long-wave and short-wave.
TOPCon solar panels are not affected by the weather, and the power generation is guaranteed continuously, while the power generation of PERC is seriously affected by the weather.
