Selective Laser Heating Applied To Detach Backsheet In PV Recycling

Key takeaways:

• University of Virginia researchers demonstrated laser-assisted methods for removing backsheets from end-of-life PV modules

• Infrared laser heating softens the EVA encapsulant, enabling separation of the backsheet while preserving glass and silicon cells

• Laser-created channels can accelerate solvent penetration, reducing module delamination time using the solvent method from several days to under 30 minutes

The use of lasers in recycling end-of-life PV modules is gaining momentum. Researchers worldwide are working on using lasers, particularly to efficiently recover materials such as glass, cells, and silver with minimal or no damage.

Researchers from the Department of Electrical and Computer Engineering at the University of Virginia recently published their work on using lasers to effectively remove backsheet from end-of-life PV modules.

Backsheets typically range from 0.3 mm to 0.5 mm in thickness and comprise multiple layers, with PET sandwiched between Tedlar (PVF) sheets, PVDF-based layers, or other fluoropolymer composites. To remove these sheets effectively, a configured continuous-wave IR laser can be directed from the front side to selectively heat the silicon area and soften the encapsulant, EVA. This softening reduces adhesion between the silicon and the encapsulant, enabling detachment without damage to the cells.

At a laser wavelength of 1,070 nm, where both glass and EVA are transparent, silicon absorbs the light and heats up. They tested different laser power densities and exposure times and found that 28-30 W/cm² at 8-12 s was optimal for effective softening and delamination. At 28 W/cm² after 12 s, the silicon cell reaches approximately 225°C, while the EVA-backsheet interface reaches 205°C, and the outer surface of the backsheet 190°C. Using the combination of this power density and time, some EVA residues are still observed after backsheet detachment. These can be removed with laser ablation at 28 W/cm² for 5-10 s. After the complete removal of the backsheet and any residue, I-V performance measurements in certain regions of the cells show no deviation from those taken before laser treatment.

Under laboratory conditions, the approximate cost of processing a module is around $0.22, according to the researchers. This is for a 30-minute cycle per module. Dr. Mool C. Gupta, corresponding author of the paper, said, “The process can be optimized for faster delamination using higher laser power or multiple lasers.” He further added that the team is working with an industrial company to commercialize this process. The complete research article, titled Laser removal of silicon solar cell backsheet while preserving tempered glass and silicon wafer, can be accessed here.

As part of the research, the use of lasers was also explored to ablate and create channels in the backsheet. These channels facilitate the penetration of chemical solvent into the EVA-backsheet interface and accelerate the detachment, in the solvent-based approach.

Laser is used to uniformly perforate the backsheet via ablation, thereby creating channels for solvent penetration. This approach can delaminate the module in 10-30 minutes compared to the conventional solvent method, which takes several days.

In a recent press release, the Netherlands Organization for Applied Scientific Research (TNO) announced that it has developed a  laser-based PV recycling method and has already scaled it up for industrial use.