Acid Leaching of CIGS Solar Cells: A Focus on Silver and Indium
Authors: Teknetzi Ioanna , Ebin Burçak 
Published: February 2024

Solar electricity from photovoltaics (PV) contributed to about 4.5 % of the global electricity production in 2022 and is expected to continue increasing by 2050 [1]. Therefore, both manufacturing waste and end of life waste are expected to reach a considerable volume in the upcoming decade. However, recycling methods for this type of waste are still at a very early stage of development. The thin film CIGS PV technology achieves high energy conversion efficiencies [2] and it is also used in promising newer technologies, namely multijunction solar cells. However critical elements like indium (In) and gallium (Ga), and often valuable silver (Ag) as conductive grid, are essential for the achievement of such efficiencies. Therefore, environmental, economic and resource depletion reasons demand the recovery of these elements.
In industrial scale, there is currently no established recycling activity on CIGS containing materials. In lab scale, research mainly focuses on recycling of pure CIGS material production waste, which are free of any other element except for their 4 constituent elements in contrast to the real solar cells or PV. However, monitoring and controlling the impurity levels in the recycling of complex waste is of outmost importance, since the higher the purity of the recovered materials, the more economically viable their recycling business becomes. At the same time, the existing literature on the recycling of pure CIGS waste suggests the use of strong mineral acid solutions and high temperatures for the recovery of their elements. The case is similar for Ag, with some studies on its recovery only from another PV technology, namely silicon PV, being available and describing similarly harsh recovery conditions. Although such conditions can be efficient in many cases, they are not environmentally friendly and can be costly for the industry as well. To the best of our knowledge, the only available research work on the recovery of valuable elements from CIGS solar cells (Ag, In and others) using more benign recovery conditions compared to the ones that have been investigated so far (i.e. lower acid concentrations and room temperature), is our recent research work on leaching with nitric acid (HNO3) [3].
In our current work we investigated the leaching of flexible CIGS solar cells with a stainless steel substrate production waste at room temperature and acid concentrations no higher than 2 M. The leaching efficiency of Ag and In was studied under different leaching times, acid leaching agents and concentrations, as well as geometrical surface area to liquid ratios (A:L). The elemental composition of all the elements that were likely to be present in the solar cell was measured in all the leachates with ICP-OES. The solid materials left on the cell’s surface after the treatment were characterized in terms of morphology and elemental composition with SEM-EDS, when necessary.
The results proved that the choice of the leaching agent plays an important role in the recovery of the different elements present in the cell. It was also confirmed that higher acid concentrations achieve higher leaching recoveries for the same leaching conditions, as expected. As far as the A:L ratio is concerned, in many cases, higher A:L ratios achieved higher efficiencies, probably due to better oxygenation of the solutions.
An example of the efficiency of the method is presented in Fig. 1, in which the Ag grid line of the untreated sample (Figure 1a, b) looks white under SEM due to the presence of Ag particles. After the acid treatment of the cell with 2 M HNO3 for 24h, the Ag particles of the Ag grid lines have disappeared, making the line look black (Figure 1c).
In total, two important conclusions were drawn: a) selective leaching of various elements for achievement of higher purity products is possible by adjusting the leaching parameters and b) a complete recovery of Ag is possible within one day with more environmentally friendly conditions than the ones suggested so far.

References

[1] International Energy Agency; Solar PV, 2023, available at: https://www.iea.org/energy-system/renewables/solar-pv#tracking
[2] Fraunhofer institute for solar energy systems; PHOTOVOLTAICS REPORT, 2022, available at: https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovoltaics-Report.pdf
[3] I. Teknetzi, S. Holgersson and B. Ebin, Valuable metal recycling from thin film CIGS solar cells by leaching under mild conditions, Solar Energy Materials & Solar Cells 252 (2023) 112178, https://doi.org/10.1016/j.solmat.2022.112178

Acknowledgement: This work was funded by the Swedish Energy Agency [49349-1]

Figure 1