Reusable crucible materials and coatings for photovoltaic silicon applications
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Currently, more than 90% of commercial solar cells are made from crystalline silicon. The crystalline silicon materials are classified into two types: monocrystalline and multi-crystalline silicon. Multi-crystalline silicon ingots are made by the directional solidification method, which controls the growth of the material in a specific direction. Typically, the silicon feedstock is charged to a crucible or the melt is poured into the crucible before the solidification step. Replacing conventional single-use crucibles with reusable crucibles will reduce the cost and the waste of the production process. However, introducing reusable crucibles on an industrial scale requires a thorough understanding of the interactions between crucibles, coatings, liquid silicon, and furnace atmosphere. This thesis aims to examine the behavior of different crucible materials and coatings in contact with liquid silicon under several conditions for more sustainable silicon solidification processes. Two crucible materials were investigated in this study; silicon nitride and graphite. Silicon nitride crucibles showed the potential to replace single-use silica crucibles. The durability of silicon nitride crucibles was tested at different temperatures in an oxidizing atmosphere. It was shown that the heat treatment step limits the lifetime of the silicon nitride crucibles. To eliminate the heat treatment step, an alternative method was developed by adding colloidal silica to the coating slurry. The proposed coating technique showed better non-wetting properties during silicon melting compared with the conventional heat treatment method. The influence of oxygen content in the silicon nitride coating on the molten silicon behavior was studied in detail. A semi-quantitative model was developed to describe the oxygen depletion from the coating during the melting process. The fundamental investigation and modelling of oxygen depletion for oxygen-free crucibles enhanced the understanding of the coating degradation mechanism and its impact on the molten silicon wetting during ingot growth. The utilization of graphite materials in silicon crystallization processes needs careful consideration of the interactions between the graphite and the coating components, which can negatively affect the quality of silicon ingots. In this work, different coating configurations were proposed for reusable graphite crucibles. The coatings' wettability and interactions with graphite were investigated by in-situ melting experiments. The two-layer coating approach revealed a considerable improvement in the non-wetting behavior, a decrease in the coating degradation rate, and a decrease in CO evolution during melting. The best performance of the coating was achieved by optimizing the thickness and oxygen concentration of the proposed coating methods. Moreover, the non-wetting properties of the coating are significantly improved by adding silicon oxynitride to the coating. The influence of different gas atmospheres on the coating stability and liquid silicon behavior was investigated via in-situ melting experiments. The impact of the morphology and the growth of nitride and carbide layers on the wettability was elucidated in detail. The results were supported by Raman analysis and thermodynamic calculations. It was shown that controlling the composition of the furnace atmosphere may improve the performance of the coating and extend the lifetime of the crucibles.