Recycling of silicon solar panels through a salt-etching approach

We report a simple salt-etching approach to recycle Ag and Si from end-of-life Si solar panels without using toxic mineral acids and generating secondary pollution.

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      We develop a simple and alkaline-based PV recycling system that uses alkali to recycle Si, Ag, Cu, Pb, and Sn by etching the surface SiNx, SiO2, Al, and Al2O3 of Si cells and Pb-Sn oxides of the oxidized solder (Fig. 1). In addition, the residual alkali can be fully utilized for absorbing the HF gases released by the pyrolysis of the Tedlar/PET/Tedlar (TPT) back sheet. Using this approach, we have greatly simplified the recycling process and minimized the toxic waste generation, as well as reduced the energy consumption.

 Our planet needs solar energy to drive all systems from nature to human activities. The solar radiation that strikes Earth in just one hour is equivalent to the amount of energy currently used annually by humans. Since solar energy is vast and clean, the efficient use of solar energy to drive human activities has been pursued for a long time. Many approaches have been tried to mimic the photosynthesis of plants that convert solar energy into organic matters and store the energy in the form of chemical bonds. Inspired by the nature, the observation of photovoltaic effects gives us confidence to make artificial solar devices to convert solar energy to electrical energy. Among many kinds of semiconductor materials, Si stands out to be used as solar cells because of its Earth’s abundance and manufacture innovation. Currently, Si solar cells dominate over 95% of the market and are projected to increase with the goal of mitigating the climate change associated with burning fossil fuels. The estimated amount of waste Si solar modules will reach more than 78 million tons by 2050, which requires a considerable amount of Si, Al, Ag, Cu, Pb, Sn, etc. For example, the consumption of Ag will reach 90 000 tons that exceeds the current production. In this case, the key material supply could be a limit to the sustainable development of solar cells. Since solar cells have limited lifetime, recovering key elements from end-of-life solar cells rather than from raw minerals are needed to close the materials cycle and make the photovoltaic industry sustainable. Therefore, the recycling of Si solar cells is necessarily needed.

However, the recycling of Si solar cells is still at its infant stage and meets several challenges: 1) it is difficult to remove the Si3N4 layer to obtain pure Si in a simple and green way, 2) the separation of Ag from the Si wafer using HNO3 and HF releases toxic NOx and HF gases and waste acids, 3) comprehensive recycling all key elements is still absent, and 4) current methods have high energy consumption and environmental footprint. Thus, we need to develop novel methods to achieve the clean recycling of critical minerals from end-of-life Si solar cells. To this end, we develop a molten-salt etching approach to etching Si3N4 layer at a fast rate and simultaneously separating Ag wires from the Si wafer (Fig. 2), along with an electrochemical way to repurpose Cu, Pb and Sn from the waste solder strips. The mechanism underlying the salt etching method relies on the surface etching reaction between molten salt and the Si wafer. The chemistry of the rapid etching and separating (less than 3 minutes) is due to the molten alkali salts (e.g., NaOH or Na2CO3) that can selectively react with Si3N4, SiO2 and Si in a thermodynamically spontaneous manner.

We believe that it is a surprising new approach in the field of Sustainable Photovoltaics pertaining to the recycling of critical metals (e.g., Ag, Si, Al, Pb, Sn, Cu, etc.) from end-of-life Si solar cells, a finding that unlocks access to recovering critical metals that are hitherto needed for making new silicon solar cells. The close of the materials supply chain is pivotal to making sustainable solar cells that are much needed to tackle the climate change and achieve a zero-carbon planet. This paper is important because the invented molten-salt etching approach provides an efficient and clean solution to replace acids and thereby phase out the generation of toxic gases and secondary wastes. In addition, silver is recovered without dissolution-deposition-purification processes, making the recycling process more energy efficient. As the ever-increasing installation of Si solar cells, the salt-etching approach will play an important role in sustaining the materials supply for making a sustainable Si photovoltaic chain. Looking forward, the green recycling of end-of-life Si solar cells will expediate the transition from fossil-fuel-driven to solar-energy-driven society, mitigating climate change and reaching the goal of carbon neutralization.

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Solar Cells
Physical Sciences > Materials Science > Materials for Devices > Photonic Devices > Solar Cells
Physical Sciences > Earth and Environmental Sciences > Environmental Sciences > Sustainability
Green Chemistry
Physical Sciences > Chemistry > Industrial Chemistry > Green Chemistry