Directional Solidification of Multicrystalline Silicon for Photovoltaic Applications

Directional solidification is a cast-based technique in which molten silicon is solidified under a controlled thermal gradient to produce multicrystalline ingots for solar-cell fabrication. A polycrystalline structure emerges when a temperature gradient is imposed along the vertical axis of a crucible, inducing a single-directional advance of the solid–liquid interface. The process yields large ingots at relatively low cost compared with single-crystal methods, although the resulting grain structure and distribution of crystallographic defects exert a direct influence on photoconversion efficiency. Optimising the cooling rate, seed configuration and furnace atmosphere minimises the formation of detrimental grain boundaries, dislocation clusters and impurity precipitates. Oxygen and carbon impurities, originating from the crucible and furnace ambient, segregate at the solid–liquid front and become entrapped within grain boundaries and bulk silicon, thereby affecting carrier lifetime. Recent developments have focused on in situ monitoring of interface morphology, enhanced control of gas flow and impurity transport, and the integration of digital tools for process control. By refining the interplay between thermal profile, crystallisation kinetics and feedstock purity, directional solidification remains a cornerstone of cost-effective multicrystalline silicon production for large-scale photovoltaic installations.

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Technical terms

Directional solidification: A crystal-growth method in which a molten silicon charge is solidified under a controlled temperature gradient to produce multicrystalline ingots with a preferred growth direction.

Grain boundary: The interface between adjacent crystallites in a polycrystalline material, often acting as a site for defect accumulation and impurity segregation.

Impurity segregation: The preferential incorporation or concentration of foreign atoms (such as oxygen or carbon) at defects, grain boundaries or the solid–liquid interface during solidification.

Melt–gas interface: The boundary between the liquid silicon melt and the furnace atmosphere, where evaporation and dissolution of volatile species govern impurity transport.

References

1. Silicon Solar Cells: Trends, Manufacturing Challenges, and AI Perspectives. Crystals (2024).
2. Effect of Argon Flow on Oxygen and Carbon Coupled Transport in an Industrial Directional Solidification Furnace for Crystalline Silicon Ingots. Crystals (2021).
3. Analysis of grain structures and impurity distribution in mc-silicon grown by directional solidification: Computational and experimental approach. Applied Surface Science Advances (2022).
4. X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation. Crystals (2020).