(Invited) Sustainable Recycling of Metals from Solar Cells

Abstract

The share of solar energy is increasing worldwide, leading to a widespread use of solar panels. Knowing that the 80 Mt of waste solar panels will be produced by 2050, sustainable processes must be designed in order to deal with the inevitable increase of waste solar cells in the coming years.The main advantage associated with recycling is to avoid the inevitable dispersion of hazardous materials in the environment occurring when the solar cells are landfilled. It also reduces the need for primary mineral resources and may reduce the supply risk for important components like silicon wafers, for which the production is concentrated in a few countries. The recycling process applied plays a major role to allow reducing the carbon footprint of solar cells, advocating in favour of sustainable practices for solar cells recycling.Mono-crystalline solar cells representing 90% of the market, it is therefore this kind of solar cells that are more likely to reach their end-of-life soon. Mono-crystalline solar cells are made of an aluminium frame surrounding the solar module. Underneath a glass layer, layers of ethylene vinyl acetate (EVA) encapsulate the solar cell. The solar cell in itself is made of a silicon wafer with aluminium and silver electrode as well as a silicon nitride anti-reflective coating. Recycling of the metals, particularly silver, is the main economic driver for solar cells recycling.Recycling mono-crystalline solar cells involves a mechanical removal of the aluminium frame and the junction box prior to removal of the glass and EVA layers. Different process were evaluated to get rid of EVA. Solvent-assisted dissolution allowed EVA dissolution but it requires the use of harmful solvents and often leads to breakage of the solar cell. Pyrolysis allows to recover the wafer intact while removing the glass and polymers layers. Provided that the thermal treatment is applied under an inert atmosphere, EVA decomposition leads to relatively non-toxic chemicals such as acetic acid. Recent developments include laser assisted would lead to efficient and straightforward removal of EVA.The removal of metals from the solar cells involves the use of harsh chemicals such as hydrofluoric acid for the removal of silicon nitride, nitric acid to solubilise silver and sodium hydroxide for the dissolution of aluminium. Recently, the use of aqueous brines was introduced to recover silver and aluminium from solar cells. A green and re-usable oxidising agent such as iron(III) chloride can be used to oxidise and dissolve silver, as evidenced in Figure 1, which shows a solar cells before (up) and after (down) etching, revealing the removal of the white silver lines.Using water as a solvent, silver dissolution is impossible due to the low chloride content. By increasing the chloride content, oxidation of silver and its dissolution becomes possible due to the formation of iron-chloride and silver-chloride complexes which have a different redox behaviour and a higher solubility. Overall, silver is completely dissolved in about 10 minutes without using any acids or harmful chemical. Silver can be easily recovered by precipitation or cementation, while the oxidising agent can be regenerated using oxygen from air or via electrolytic means. The objective of this presentation is to give an overview of the current recycling options and provide some guidance on the best and more sustainable practices for the complete recycling of solar cells. The choice is guided by techno-economic analysis and life cycle assessment as well as experimental demonstration of the recycling on real waste solar cells. Figure 1