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How to reduce the defective product of PERC cell EL? 2019-09-05
During the production process, PERC cell passivation film damage and various EL defects caused a serious drop in yield. This paper explores certain optimization solutions through the production process, equipment and production management of PERC cell.

1PERC cell and conventional cell

Conventional batteries use a conventional back-field cell(BSF) structure, which has inherent limitations, and as the cell efficiency increases, the limitations become more apparent. Applied to BSF cell back-field metal aluminum film can not reduce the backside recombination speed, such as below 200cm / s. Only 60%-70% of the infrared radiation of the metal aluminum back layer can be reflected back.

PERC cell using PERC technology require a backside passivation solution based on conventional back field (BSF) technology. In a specific implementation, a back passivation film needs to be deposited, and then the film is grooved to achieve back contact. By attaching a dielectric passivation layer to the back of the cell, photoelectric loss can be reduced and cell efficiency can be improved.

2PERC cell EL defect analysis

2.1 Local scratches

In the PERC cell preparation project, local scratch marks are inevitable. For the passivation film with a very good back surface, the scratch marks cause the back surface recombination rate to locally decrease. These scratches may not affect the overall efficiency, but from the EL photo Seeing there will be scratches and shadows, which are classified as defective products. Some enterprises may have 20% to 30% of EL defective products due to scratches. The cause of scratches is related to the smashing of the film during the process, the operation of the worker, the operation of the process equipment, etc., which is difficult to avoid.

2.2 Process pollution

In the PERC cell preparation project, the cell is not cleaned, the suction cup is in contact, the bar is in contact, the basket is in contact, the person is in contact, the dust is contaminated, and the like.

Analysis of Causes of EL Defects in 3PERC cell 

3.1 PERC cell process and related equipment

The main process of PERC cell includes the following aspects:

The first is to cut the damage and the cashmere to form the gold suede. The main equipment used in this process is Jiejia Chuangsheng trough type fleece;

The second is diffusion to generate PN junction. The equipment involved in this process is Tempress diffusion furnace, R2D automation, Jiejiachuang low-pressure diffusion furnace;

The third is edge etching and dephosphorization of silicon glass, this part requires instrument SCHMID etching machine, SCHMID automatic movement instrument;

Fourth, back passivation deposits aluminum oxide film and silicon nitride film on the back side of the silicon wafer. This part requires MeyerBurger's one-piece machine coating machine and Robote Automated Instrument;

The fifth is the deposition of silicon nitride film on the front side of PECVD. The main instruments used in the process include CT tube coater and Baumann automation.

Sixth, the back laser opening realizes partial contact between the backside paste and the silicon material, and the process is used as a Dier laser instrument;

Seven is screen printing, this process uses ASYS printing machine;

Eight is drying, sintering, light plus annealing process, this process uses Despatch sintering annealing furnace; nine is test analysis, the main instruments are ASYS sorting machine, BERGER testing machine.

3.2 Main causes of scratches and pollution

Through the practice and operation analysis, it can be known that the PERC cell is scratched and contaminated:

Cashmere process: incomplete damage, cleaning residue, personnel contact, drying pollution, dust pollution, oil stains, etc.; inserts,

Rewinding process: belt contact, bar contact, flower basket contact, personnel contact, dust pollution, etc.;

Diffusion process: friction between suction sheet, boat card slot and silicon wafer; etching process: roller scratch, cleaning residue, belt contact, guide bar contact, flower basket contact, personnel contact, drying pollution, dust pollution, oil stain, etc.;

Back passivation process: suction cup contact, belt contact, guide bar contact, flower basket contact, personnel contact, dust pollution, etc.;

PECVD process: suction cup contact, belt contact, guide bar contact, flower basket contact, boat contact, personnel contact, dust pollution, etc.;

Laser drilling process: suction cup contact, belt contact, guide bar contact, flower basket contact, dust pollution, personnel contact, etc.;

Screen printing: belt contact, bar contact, flower basket contact, personnel contact, dust pollution, slurry contamination, etc.;

Drying, sintering and annealing processes: personnel contact, dust pollution, furnace belt pollution, oil pollution, etc.; test analysis: suction cup contact, belt contact, guide bar contact, personnel contact, dust pollution, etc. In short, the main causes of scratches and pollution can be attributed to process factors, equipment factors and personnel factors.

4Research on measures to reduce EL defect of PERC cell 

For PE defects in PERC batteries, it is necessary to analyze specific problems and take targeted measures.

1. Incomplete damage to damage, strict control of the original silicon wafer inspection work, reduce the inflow of severely defective wafers; optimize the ratio of chemical liquid and process time, and remove wafer damage.

2. For roller scratch defects, regularly check the wear of the bevel gear, the position of the upper and lower rollers, the tightness of the moving parts of the roller, and if necessary, timely maintenance and repair; regularly check and optimize the position of the guide wheel components.

3. In the aspect of residual cleaning: optimize the spray flow rate of the cleaning tank; clean the tank filter regularly.

4. For belt contact: etch the blank to install the flipper to reduce the contact between the back of the wafer and the belt; regularly check the position of the wafer into the box, the position of the sensor, reduce the friction between the belt and the wafer; clean regularly, better belt , reduce belt silicon powder, slurry contamination.

5. Guide strip contact: optimize conductor position, reduce silicon wafer contact frequency; regular cleaning, reduce conductor silicon powder, slurry contamination; regularly replace wear-resistant tape to reduce friction with silicon wafer.

6. Flower basket contact: regularly clean the flower basket to reduce the contamination of the flower basket; regularly optimize the automatic position, reduce the friction between the wafer and the basket; regularly check the size of the basket and promptly clear the deformed flower basket.

7. Suction cup contact: Clean the suction cup regularly to reduce the contamination of the suction cup; optimize the automatic position regularly to reduce the friction between the suction cup and the silicon wafer.

8. Contact with the boat: Regularly clean the graphite boat and frame to reduce the contamination of the suction cup; regularly optimize the automatic position and reduce the friction between the graphite boat, the frame and the silicon wafer.

9. Personnel contact: Strengthen production line management, avoid all contact pieces and stained sheets from flowing into the next process; strengthen operation management and reduce scratching and staining of silicon wafers caused by personnel handling and inspection.

10. Drying pollution: Regularly clean the dry area debris to reduce dust pollution; regularly replace the filter element, clean the pipe, air knife, roller and so on.

11. Furnace belt pollution: Regularly clean the furnace belt, reduce road oil pollution; regularly check the deformation of the road belt, adjust and replace it at any time.

12. Dust pollution: After etching, the silicon wafer is transported in a back passivation process to reduce the contamination of the silicon wafer during transportation; the back passivation silicon wafer has not been stored, and must be stored in a constant temperature, positive pressure nitrogen sealed cabinet, and time It should not be too long; the integrated coating machine should be thoroughly and completely maintained to avoid the dust retention in the chamber; strengthen the workshop management, reduce the air dust particles in the workshop, and ensure the cleanliness and compliance.

5 Conclusion

In order to reduce the damage of PERC cell passivation film and various EL defects, PERC cell production process must start from the details, through the formulation of detailed, perfect, scientific process plan, equipment management, maintenance system, as well as personnel operation and process. Detailed, strict management, and efficient execution, and finally ensure that PERC cell production process, production equipment, production environment standardization, stabilization, and cleanliness can be realized.
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