Recycled kerf for mono silicon feedstock

REC Solar produces feedstock for silicon solar cells using a unique, metallurgical production process based on recycling of fines from the wafer slicing process (“kerf”). This material is established as feedstock for production of p-type monocrystalline ingots and wafers for low carbon footprint markets. The recent transition to Ga as the primary dopant in p-type PERC cells, and the ongoing market transition to n-type cell technologies, introduces challenges related to the inherent boron concentration in REC feedstock. This project has focused on module degradation mechanisms that depend on the properties of the feedstock, one related to B and O, the other to H introduced in cell processing. Degradation is counteracted with processes using combinations of elevated temperature and illumination/current. In a previous project we found degradation and regeneration kinetics to depend on the concentration of the different dopants. A main finding was that it is possible to regenerate compensated material to the same level as single dopant references. The degradation phenomena have been studied using sets of wafers with differing dopant compensation and pretreatment. Time series of minority carrier lifetime are measured under conditions that accelerate degradation and regeneration. Kinetic models were used to separate the contributions of each degradation mechanism. A detailed understanding of the degradation and regeneration phenomena and how they depend on dopant composition enables setting of tolerance limits of impurities and guidelines for optimal treatment of wafers made with REC feedstock, both for p-type and n-type materials.

The project led to improved understanding of the material properties and dopant interactions in dopant compensated E2M-feedstock. Insight into the degradation kinetics enables customization of the stabilization process, ensuring optimal performance of the material. By both reducing the impurity levels and mitigating their performance limiting effects it was demonstrated that E2M can be a viable feedstock for advanced solar cell types.

In this project recycled wafering fines (“kerf”) will be established as a cost-effective, circular, low CO2 footprint primary silicon feedstock material, called E2M, for future high efficiency p- and n-type cell technologies. The recent transition to gallium as the primary dopant in p-type PERC cells, and the forecasted market transition to n-type cell technologies, introduces new challenges related to the inherent concentration of trace amounts of dopants in E2M solar grade silicon that will be addressed in this project. There are many sources of module degradation, but there are two types associated with the wafer and silicon feedstock quality which are triggered under illumination in the field: BO-LID and LeTID. Both mechanisms have been studied extensively in p-type boron doped silicon, but there is limited information available in the literature on the behavior in compensated materials. We aim to establish fundamental understanding of the physical properties of this new class of solar grade silicon focusing on performance limiting impurities. The project will focus on both reduction of the impurity levels through improved refining processes, and by developing strategies and processes to mitigate their effect. New and improved thermal wafer cleaning processes will be developed to reduce the concentration of performance limiting metallic impurities in emerging low temperature cell processes.