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Scientists in the Netherlands have proposed a new test system for recycling silicon from end-of-life photovoltaic panels. Their methodology helped create different categories of wafers for recycling silicon destined for the production of new ingots, but it also showed that most recycled silicon in the near future will come from p-type products, which are unlikely to be reused in a market now dominated by n-type modules.
A research group coordinated by the Netherlands Organisation for Applied Scientific Research (TNO) has investigated how clean wafers or wafer fragments recovered from end-of-life photovoltaic modules could be reused for the production of new crystalline silicon ingots and has found that gallium-doped wafers could be particularly suitable for this purpose.
The scientists explained that silicon from discarded wafers must be made by removing any contamination on its surfaces, which would put it back into the category of high-purity material. “The main contaminants are the dopant, oxygen, carbon and maybe some nitrogen,” lead author of the research Bart Geerligs told pv magazine . “We looked at it primarily from the perspective of the dopant and resistivity control, and to a limited extent also from the perspective of other remaining contaminants.”
In the study “ Potential for Recycled Silicon Solar Cells as Feedstock for New Ingot Growth”, published in Progress in Photovoltaics , the researchers explain that their analysis addresses potential technical and economic constraints relating in particular to dopants and impurities. They also expect that significant volumes of silicon, particularly from p-type wafers, will be recoverable from around 2040 onwards, with the boron-doped and gallium-doped markets split roughly equally.
The research group also created a methodology to separate n-type and p-type modules, and boron-doped panels from boron- or gallium-doped p-type ones. It established, for example, that if the modules solar cells are polycrystalline, they are necessarily p-type doped with B. "To our knowledge, no n-type modules based on polycrystalline silicon have been commercially produced," the researchers say.
They also created a separation between wafers that do and do not have front-side metallization. They also said that the voltage of all modules except those based on interdigitated back-contact (IBC) cell technology should be identified, and that a visual inspection of the backside of all cells should be performed. “The principle for inspection is then that all Al-BSF and PERC p-type industrial cells have Al back-side metallization combined with local silver pads for soldering the interconnect ribbons, and n-type industrial cells have no such combination,” they specified.
The team explained that the entire testing scheme could be bypassed if a label on the discarded panel contained useful information. “For example, one could document that a module contains HJT (n-type) cells or that it is based on IBC cells from a manufacturer such as Sunpower or Maxeon,” they further explained. “It would also be very useful if PERC modules visibly displayed a production date because from before 2019, this would imply boron doping, and after 2022, it would imply gallium doping of the wafers.”
“This scheme would result in three material streams,” Geerligs explains. “These are n-doped cells, boron-doped cells with p, and a stream of monocrystalline PERC cells that could be doped with boron or gallium.”
The scientists concluded that reusing p-type wafers as feedstock for new p-type ingots will not be economically viable, as n-type cells are now the dominant technology.
“The potential cost reduction from using recycled raw material does not seem to be sufficient to offset this fact,” they stated. “Another possibility for much higher cost-effectiveness for recycling p-type wafers may become available with perovskite-silicon tandem technology, in which case both the efficiency disadvantage compared to n-type is strongly reduced and the PERC cell performance can be improved by a poly Si emitter.” |
