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An international team of researchers has proposed a series of processes to recover silicon and other metals from recycled solar cells. Its goal is to reuse recovered silicon in the photovoltaic supply chain.
A European group of scientists has investigated the technical feasibility of physically recovering the metal contacts of solar cells extracted from end-of-life solar panels and qualifying the recovered silicon for reuse in the photovoltaic supply chain.
“This reality urgently requires the design and adoption of affordable, effective and massive recycling and reuse strategies for all photovoltaic components, and in particular those that consume the most energy in their manufacturing and, therefore, are responsible for the largest share.” of environmental impact: solar cells,” explain the scientists.
Demetallization
For the recovery of metallic contacts, the group used solutions of potassium hydroxide (KOH), ethanol and water to optimize the demetallization process. They tested various configurations of the solution on old monocrystalline solar cells. The front contacts of the cells were made with silver screen printing paste, as were the rear contacts, although they also contained traces of other metals, such as aluminum, iron or lead.
“The operating conditions for each experiment were the following: 105 min of immersion in 250 mL of etching solution, with a solid/liquid ratio of 0.004 g/mL, magnetic stirring and temperature between 60 ºC and 70 ºC,” explain the researchers. scientists. Pre- and post-treatments were carried out to minimize the eventual presence of impurities on the sample surface.
The demetalization experiments had three variables: different operating temperatures (60 ºC, 65 ºC and 70 ºC), different portions of KOH (10%, 15% and 20%) and different rates of ethanol (5%, 10% and 15 %). The process was carried out in a tall 250 mL beaker in which the cell fragments were introduced in a vertical position held by plastic tweezers.
“The design of experiments allows us to conclude that temperature is the most influential parameter in the weight loss and the demetallization rate, while the ethanol-temperature interaction is the most influential factor in the useful life of the minority carrier,” they noted. “Processing at a temperature of 60 ºC in a 10% KOH – 5% ethanol solution for 105 min is revealed as the best option for the recovery of pure metals while preserving the greatest amount of silicon possible.”
Recrystallization
Regarding the recrystallization process of recovered silicon, the academic started with “clean fragments of old commercial multicrystalline B-doped wafers,” which were manufactured in the 2000s. They used a Czochralski (Cz) cultivator to create an ingot, which was then cut into 79 wafers. These were subjected to phosphorus diffusion gettering (PDG) processes to improve their transport properties.
“Following successful growth and wafer slicing, an exhaustive optoelectronic characterization program was applied,” the research group noted. “It demonstrated compliance with the basic requirements necessary to manufacture new solar cells, in terms of wafer resistivity (around 1 O cm), oxygen content (around 1,018 cm3), mobility (around 1,000 cm2/ V·s) and carrier lifetimes (above the threshold of 100 µs in all cases and reaching maximum values ??of 350 µs).”
The group used the recovered materials to fabricate phosphor/aluminum back surface field (P/Al-BSF) solar cells. The structure included evaporated metal contacts, with titanium/palladium/silver (Ti/Pd/Ag) for the front contact and Al for the rear. Finally, IV characteristics were obtained under single-sun to four-cell illumination and compared to a reference.
“From our results, we conclude that the main limitations in device performance observed in those manufactured from recrystallized material and demetalized wafer fragments are not attributable to the degradation of material quality that is eventually incurred during the recovery and utilization stages of the silicon substrates, but rather to failures during the manufacturing of the cells,” the team concluded.
Their analysis was presented in the article “ Validation of recycling processes for demetallization and recrystallization of silicon solar cells ,” published in Solar Energy . The research was carried out by scientists from the Complutense University of Madrid, the Polytechnic University of Madrid and the Leibniz Institute for Crystal Growth (IKZ) in Germany. |
