Solar panel recycling method that recovers cell metals for upcycling

• A team from Australia’s University of New South Wales (UNSW) in Sydney reported being able to recover silicon, silver and tin from PV cell layers.
• The method recovered the majority of silver (Ag) (70 wt%) and the processed silicon (Si) exceeded 99.7 wt% purity.

The method is described in “Microrecycling of waste solar cells via an in-situ fluorine-generating thermal treatment for high purity silicon recovery,” published by Resources, Conservation and Recycling.

The micro-recycling approach enables the selective recovery and transformation of valuable materials at the microscale, according to Rasoul Khayam Nekouei, corresponding author of the research, rather than relying on traditional bulk recycling methods.

“It also refers to the microscale characterisation techniques used to investigate the recovery mechanisms of valuable metals,” he explained.

In the study, the manual separation of the junction box, cables, and aluminium alloy frame is followed by the module glass delamination via a hydrothermal delamination method, which was developed by the study authors in earlier research.

Next, the waste cell material was subjected to a medium-temperature heat treatment at 550 C, effectively removing the ethylene vinyl acetate (EVA) encapsulants and polyvinyl fluoride (PVF) backsheets. The “medium-temperature in-situ F-producing thermal treatment” was able to eliminate the corrosion resistance imparted by the top layer of the silicon (Si) cell, according to the team.

“The biggest surprise was discovering that fluorine, which degrades this corrosion-resistant layer, was already present in the solar panel’s backsheet. This meant that the problem and its solution were right next to each other,” Nekouei told pv magazine.

In the second step, both the heat-treated silicon (HT-Si) and untreated (UT-Si) fragments underwent chemical etching using basic 30 wt% potassium hydroxide (KOH) and nitric acidic (20 v% HNO3) media at 80 C. Surface roughness and elemental analyses supported decisions about how long to perform etching to remove aluminium (Al) from the HT-Si and UT-Si fragments.

The two-step etching removed Al impurities within 3 min and “facilitated the liberation of 70 wt% of Ag strips from the wafers while avoiding Si loss. The remaining Ag and Sn were extracted using the second etching stage via an acidic media,” according to the researchers.

The team said it was worth noting that the Si purity of the back side of UT-Si fragments remained around 77 wt%, while that of HT-Si fragments exceeded 99 wt%, despite undergoing only 3 minutes of KOH etching.

The etching process not only prevented Si loss but also facilitated the mechanical disengagement of over 70 wt% of the Ag strips.

It was noted that the medium-temperature heat treatment “can significantly reduce the duration of both etching steps, facilitating the recovery of high-purity Si fragments (≥99.7 wt%).”  Furthermore, for lead (Pb) removal, 10 min of acidic etching proved sufficient, although longer durations showed no adverse effects on its recovery, according to the team.

As a demonstration of upcycling into high-value products, the team processed purified HT-Si fragments (P-HT-Si fragments) with 99.7 wt% purity to produce beta silicon carbide (β-SiC) wafers. Analysis revealed that the SiC possessed a mix of sub-micron particles, “nanoparticles, and nano-whiskers coated with a SiO2 shell of thickness 2–3 nm.” The resulting material was deemed suitable for microwave absorption applications, such as gas sensors.

The team concluded that its “micro-recycling method” offers a “sustainable path toward minimising waste in recycling spent photovoltaic solar cells.”

Looking ahead the research team will explore a wider range of waste solar panels, optimising the process to enhance recovery efficiency and material purity, and exploring potential commercialisation opportunities, according to Nekouei. “Besides, we plan to utilise the purified recovered silicon in advanced applications, such as sensing and energy storage.”

Nekouei added that commercialisation would entail further investigation, to assess feasibility, evaluate market viability, the competitive advantages, and evaluate potential business models

Author: Valerie Thompson

This article was originally published in pv magazine and is republished with permission.