Breaking the Silicon Dependency

The European Union’s pursuit of a green energy transition is currently tethered to a significant geopolitical vulnerability. Since 2014, silicon has been designated as a Critical Raw Material (CRM) due to its high economic importance and a supply risk that is officially classified as “above the limit.” Today, the EU relies on imports for 63% of its silicon needs, with China alone controlling 66% of the global market. Crucially, silicon remains “still irreplaceable” across multiple sectors; beyond photovoltaics (PV), it is a fundamental component in smartphones, computers, and batteries upon which modern society relies. Reducing this dependency is not merely an environmental objective but a strategic imperative for European industrial sovereignty. Failure to secure a domestic, circular supply chain threatens the stability of the European Green Deal and the EU’s leadership in the global renewable energy landscape.

The Metallurgical Breakthrough: Carbon-Free Silicon Production

The cornerstone of the RESiLEX metallurgical strategy is the SisAl process, a sustainable alternative to traditional smelting. While conventional methods rely on carbon as a reductant—inevitably releasing substantial volumes of CO₂ and NOx—the SisAl process utilizes an aluminothermic reduction of molten silicates. This shift shifts the industry from endothermic, carbon-heavy reactions to an exothermic process that utilizes secondary aluminum to produce high-value silicon. This metallurgical breakthrough transforms industrial “waste” into a strategic asset, providing the essential precursor for high-purity solar applications.

Mechanics of the SisAl Process

• Secondary Raw Materials: The process revalorizes waste streams, including aluminum scrap, silica fines, aluminum dross, and Si-kerf (the silicon sawdust generated during wafer processing).
• Solid-Liquid Reaction Advantage: Traditional processes are limited by solid-vapor reactions, which cannot handle fine materials because they lead to excessive material loss. The SisAl process utilizes solid-liquid reactions, allowing for the successful inclusion of fines that would otherwise be discarded.
• Environmental Differentiators: By eliminating the use of carbon as a reductant, the process removes direct CO₂ emissions from the production cycle and significantly reduces NOx output, aligning the manufacturing process with the environmental goals of the energy products it creates.

The Purification Journey: From 2N to the 5N Milestone

Raw recycled silicon, while a metallurgical success, does not possess the purity required for high-performance photovoltaic cells. To reach solar grade, the material must undergo a rigorous purification journey to eliminate contaminants such as boron, phosphorus, and metallic impurities. Led by the Norwegian University of Science and Technology (NTNU), this phase tested competing refining methodologies to determine the most effective path toward industrial-scale purity.

Comparison of Refining Methodologies

• Directional Solidification: Tested in a Crystalox DS250 furnace, this method proved insufficient for solar requirements. While it reached 3N purity (99.98%) at the bottom of the ingot, the central sections remained at a lower 2N level, demonstrating that this method could not consistently reach the targets necessary for high-efficiency cells.
• Czochralski (Cz) Growth Process: This method proved to be the project’s breakthrough. By melting a blend of recycled and electronic-grade silicon and employing a manual “fishing” technique to remove visible surface contaminants before pulling the crystal, researchers achieved a dramatic leap in quality.

The results of the NTNU pilot trials represent a landmark for the circular economy. Utilizing the Cz method, the team produced 22kg to 30kg ingots that achieved a purity level of 99.999% (5N) for the first 70% of the ingot. With total impurities below 10 PPM, this material moved from being a laboratory curiosity to a viable candidate for industrial solar cell fabrication.

From Lab to Pilot: Validation at the Ingot and Wafer Level

To validate the industrial suitability of the recycled feedstock, the high-purity material was transitioned to the French Alternative Energies and Atomic Energy Commission (CEA) for an “In-Got Scale Assessment.” This stage served as the ultimate proof of concept, testing whether recycled silicon could meet the rigorous specifications required for high-efficiency heterojunction (HJT) solar cells.

Technical Snapshot: Blending and Performance

• Stress Test (1.8% Blending): Simulation conducted on lower-purity material (2N-4N) containing boron levels approximately 200 times higher than solar grade. This validated the material’s behavior under extreme impurity conditions.
• Strategic Milestone (11–12% Blending): Utilizing the actual 5N recycled material, CEA achieved a remarkable milestone by reaching blending rates of 11% and 12% in 30kg ingots.
• Electrical Targets: The resulting ingots successfully achieved n-type conductivity with resistivity targets between 0.8 and 1.6 Ω.cm, fulfilling the precise requirements for HJT cell technology.

The “So What?” of this validation is found in the wafering process. CEA transformed these 30kg ingots into over 700 M2-size wafers. Critically, no mechanical issues or structural instabilities were observed during the sawing process, proving that the recycled material possesses the integrity required for standard industrial manufacturing. This volume of wafers provides the necessary foundation for the upcoming performance evaluations to be done by CEA.

A Resilient European Solar Value Chain

The collaboration between NTNU and CEA has successfully demonstrated that the bridge between metallurgical waste and high-tech manufacturing is not only possible but industrially feasible. By proving that silicon scrap and secondary aluminum can be transformed into 5N-purity solar wafers, the RESiLEX project has provided a blueprint for a more resilient, circular European solar economy.

Strategic Takeaways

• Circular Business Models: The project proves that waste streams like Si-kerf and aluminum dross can be revalorized, creating new industrial synergies between recyclers and the semiconductor sector.
• Technological Sovereignty: By mastering domestic production and purification, the EU can “keep the value of its scrap” within the continent, reducing the a consistent portion of its import reliance and insulating the energy transition from global supply shocks.
• Environmental Decarbonization: The transition to the SisAl process ensures that the production of solar components is as green as the energy they generate, eliminating direct CO_2 emissions from the smelting stage.

Learn more on the dedicated documents and videos

Factsheet #4 – Testing recycled silicon for PV applications

D3.3 – Ingot-scale assessment of the PV properties of the produced silicon