In-situ X-ray imaging and EBSD applied to the understanding of the mechanisms of solidification in silicon for photovoltaic applications.
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Date
2023
Journal Title
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Publisher
Ecole Nationale Supérieure de Technologie et d'Ingénierie. Annaba (Ex ENSMM)
Abstract
Silicon is the dominant material in the photovoltaic sector. Its production still presents industrial and scientific research challenges to improve its performance while mastering the cost. Within this context, silicon obtained through the Cast-mono method has a quasi monocrystalline structure at a reduced cost compared to other techniques of monocrystalline ingot fabrication. However, the understanding of the mechanisms of the directional solidification of silicon remains important to develop this technique and control growth defects. During this dissertation, a unique approach combining two in-situ techniques and an ex-situ technique was adopted. In-situ techniques are based on X-ray imaging: radiography and topography (Bragg diffraction imaging). Radiography is mainly used to observe the solid-liquid interface, topography to observe deformations and crystalline defects during solidification. The ex-situ technique is EBSD (Electron BackScatter Diffraction), which is used to characterize the grain structure and their crystallographic orientations. The correlation of these 3 complementary techniques allows us to improve our understanding of nucleation and defect interaction phenomena during the directional solidification of Si. The comparative analysis reveals a significant occurrence of Σ3 twin boundaries in Cast-mono samples compared to pure samples. Similarly, random angle grain boundaries (RAGB) also show a higher occurrence, possibly due to grain competition. Σ3 grain boundaries (GBs) play a role in reducing dislocation density during solidification. The presence of artificially designed grain boundaries (ADGBs) and sub-grain boundaries (SGBs) in the bicrystal seeds leads to deformations in the crystal planes, which facilitate grain nucleation during solidification. SGBs, like GBs, exhibit competition during solidification and form observable grooves in topography.