| Work Detail |
Indian scientists have produced high-purity polysilicon ingots from recycled solar cells using “spark plasma sintering” (SPS), and say they can achieve a purity level comparable to commercially available products.
Researchers from the Academy of Scientific and Innovative Research (AcSIR) and the CSIR National Physical Laboratory in New Delhi, India, have used the SPS technique to produce polysilicon ingots from solar cells recycled from end-of-life photovoltaic modules. They claim to have produced small sized ingots with purity levels of 98% to 99%.
These purity levels slightly exceed the 3N purity level, which scientists say is higher than metallurgical grade silicon. It is also “good enough” for applications other than solar cells, such as battery materials.
“We are currently working on the purification process of the recovered silicon to obtain ingots with a purity level higher than 5N, so that they can be used in the manufacture of new solar cells,” researcher Sushil Kumar told pv magazine . “This work is expected to be completed in the coming months.”
The scientists used a modified hot pressing technology known as SPS, which involves passing pulsed direct current with millisecond pulse widths, high current and low voltage through the pressing tool and sintered body. This method allows for rapid heating and short processing times, typically a few minutes. The researchers specifically applied this technique to consolidate silicon powder obtained from recycled solar cells.
They carried out the experiment with a solar module measuring 98 cm × 164 cm and weighing 17.18 kg. They used mechanical treatment to remove the aluminum frame (0.62 kg) and junction box. They then cut the remaining parts of the panel into small pieces of varying sizes, from 9.5 cm × 11 cm to 12 cm × 16 cm.
The process involved heat treating the recovered wafer pieces on a stainless steel plate at an optimized temperature of 480°C for 30 minutes in a muffle furnace, resulting in a total weight of 23.04 grams. The glass, silicon cell fragments and connecting wires were recovered, and the EVA encapsulant and backsheet were removed. To remove residual ash adhered to the cell pieces, the cell fragments were subjected to ultrasonic cleaning with hot distilled water for one hour.
The researchers then used SPS to consolidate the powder at temperatures ranging from 1,100°C to 1,200°C, well below silicons melting point of 1,410°C. Sintering cycles were carried out for a maximum of 20 minutes under vacuum conditions.
The Indian team recorded removal rates of 97.72% for aluminum and 99.90% for silver using this process. Using X-ray fluorescence (XRF) analysis, they determined that the resulting ingots reached a purity level greater than 3N.
“Although silicon recovered from waste is used in production, it is very likely that ingots of much higher purity can be easily produced using improved recovery and purification techniques,” the academics concluded. |
