Recycling of CIGS solar cells: environmentally friendly approaches for silver and indium recovery

Licentiate thesis, 2023

The increase in electricity produced by solar energy is expected to create an end-of-life photovoltaics (PV) waste problem in the following decades, while their manufacturing waste is already a reality. However, their recycling is still at a primitive stage. Among the other PV types, Copper Indium Gallium diSelenide (CIGS) thin film technology can achieve high energy conversion efficiencies, while consuming small amounts of materials. However, the use of critical, precious and toxic elements in this PV technology is a considerable drawback due to risk on sustainable material supply and environmental concerns. Thus, the waste should be treated properly.

The scientific literature on waste recycling of indium (In) and silver (Ag) from CIGS solar cells, which is very limited, suggests that their dissolution is accomplished through leaching in acid solutions with high concentrations and, many times, at high temperatures. However, such conditions are not environmentally friendly and can also be costly for the industry. Another challenge on the recycling is the contamination issues from other elements that are usually ignored when recycling of real CIGS PV is investigated.

The main focus of this thesis is the recycling of Ag and In (although other elements are studied as well) from flexible CIGS solar cells, using mild leaching conditions. Two processes were tested, both at room temperature: a) a method for leaching of the cell with nitric acid (HNO3) of relatively low concentrations for recovery of Ag and In and b) a method for recovery of Ag and Indium Tin Oxide (ITO) particles using ultrasonic (US) assisted leaching. 100% recovery of Ag and 85% of In was achieved within 24 h when leaching with 2 M HNO3 and surface area to liquid ratio (A:L) equal to 1:3 cm2/ml. However, these results were improved with the second method for a complete recovery of both Ag 95 wt% pure and ITO 70.5 wt% pure. For that, a two-step US-leaching process with 0.1 M HNO3 and A:L equal to 1:3 cm2/ml was used. Both methods offer the advantage of achieving 100% recovery of Ag using relatively benign conditions. Especially the improved US-leaching approach opens up a new path for possible direct reuse of the Ag and ITO particles in the manufacturing of new PVs, after further purification, with an impressively low need for chemicals.