The effect of initial growth interface on the grain structure in HPMC-Si ingot

Abstract

Seeding is an explored way to control the final structure of Silicon PV ingots. In HPMC-Si (High Performance Multi-Crystalline-Silicon) ingots, nucleation on numerous seed grains creates grain boundaries that can terminate the propagation of dislocation clusters. In the present work, we focus on the impact of the initial growth in G2 ingots that were prepared by directional solidification with a seed layer of poly-Si chunks. The subsequent grain structure formation was characterized by photoluminescence, metallography, and EBSD (Electron Back Scatter Diffraction). In the remaining seed region, different photoluminescence intensities are evidenced which reveals the existence of two morphologies, a genuine non-melted seed from the poly-Si chunks, and a region within the seed layer that is generated from re-solidified infiltrated molten silicon. Both grain morphologies in the seed layer have a random orientation and we evidenced that all grains at the solid-liquid interface grow by epitaxy when growth starts. As a consequence, grains in the first growth layer are also randomly oriented. The columnar grains, which grow from wider grains in the seed layer, are larger. During growth competition, these grains reach a higher solidification height compared to grains that grow on narrow seeds. Additionally, the dominant grain boundary types are RAGB (Random Angle Grain Boundary), followed by Σ3 twin boundaries in both seed and growth regions. These results give prospects to improve seed arrangement or coating for application to HPMC-Si process.