Upgraded Metallurgical Grade (UMG) Silicon: Quality, Applications and Process Economics

In this work, industrially processed silicon kerf loss (abbreviated to silicon kerf) from the photovoltaic industry is assessed as an anode material for the lithium-ion battery (LIB). The study includes both a characterization of processed silicon kerf from different sources and a comparison with commercially available nano-sized silicon (40 and 100 nm) in electrochemical testing. Such a direct comparison between these two silicon types in electrochemical testing provides a new insight into silicon kerf as an anode material. The silicon kerf particles are flake-like with varying lengths, with a mean particle size (d50) measured to ∼700 nm and a dimension of thickness of a few tens of nanometers. However, the specific surface area ranging from 20 to 26 m²/g is comparable to that of a silicon material of size ∼100 nm. The silicon oxide layer surrounding the particles was measured to 1–2 nm in thickness and, therefore, is in a suitable range for the LIB. In terms of electrochemical performance, the silicon kerf is on par with the commercial nano-sized silicon, further supporting the size evaluation based on the specific surface area considerations. Initial discharge capacities in the range 700–750 mAh/g (close to the theoretical value for the 12 wt% Si mixture with graphite) and first cycle efficiencies of 86%–92% are obtained. The cycling stability is comparable between the two materials, although the differential voltage analysis (DVA) of the galvanostatic data reveals that only the silicon kerf samples maintain silicon activity beyond 120 cycles. This study shows that industrially processed silicon kerf has characteristics similar to nano-sized silicon without reducing the size of the silicon kerf particles themselves. Considering its low carbon footprint and potentially lower cost, it can thus be an attractive alternative to nano-sized silicon as an anode material for the LIB industry.

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Original Research

04 March 2024

Effect of Mg alloying and cooling rate on the microstructure of silicon

Mengyi Zhu

, 2 more and

Marisa Di Sabatino

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Review

13 February 2024

Production of upgraded metallurgical-grade silicon for a low-cost, high-efficiency, and reliable PV technology

José Manuel Míguez Novoa

, 13 more and

Guillermo Sánchez Plaza

Cell processing impacts (in standard units) for the selected categories (ES mix scenario). (A) Climate change emissions in kgCO2eq per cell Wp. (B) Acidification terrestrial and freshwater in mol H+eq per cell Wp. (C) Land use in standard units. (D) Use of resources in kg Sb eq per cell Wp.

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations.

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