Inclusions In Multi-Crystalline Silicon Ingot For Solar Cells-Their Distribution And Development

In recent years, it was becoming more and more important to utilize solar energy through solar cells. Silicon is the presently the dominant materials for fabricating solar cells. Currently, cast multi-crystalline silicon has replaced mono-crystalline silicon as main photovoltaic materials. Understanding the properties of these impurities of mc-Si materials could help us find the way to reduce the cost of mc-Si solar cells and produce high quality mc-Si ingots.In this thesis, the phases and the distributions of inclusions of the mc-Si ingots have been embedded studied. The inclusion particles severely affect surface quality of multi-crystalline silicon wafers, and threaten the wire cutting process of the wafer production from multi-crystalline silicon ingot. Because they may cause wire broken in the cutting processes. Scanning electron microscope associated with energy spectrometer for characteristic X-ray, 3D digital microscope and X-ray diffraction have been used to analyze the inclusions in a directionally solidified multi-crystalline silicon ingot. The inclusions were collected by dissolving silicon samples from different positions of the silicon ingot, and extracting the un-dissolved precipitations. The results show that, there are two types of inclusions:β-SiC andβ-Si₃N₄, with the amount of the former relatively larger. Their morphologies are remarkably different, the SiC inclusions appear as irregularly shaped polyhedron particles, while the Si₃N₄ inclusions appear as straight rods. The inclusions are highly concentrated in the top layers of the ingot, rarely seen in 10 mm below surface. However, occasional appearance of SiC particles of larger than 100μm in the internal part of the ingot is still found.Experiments show that: For the metallurgical silicon, the reasons of the occasional appearance of SiC particles in its interior are not the SiC particles subsiding in melting silicon. The emergence in its interior may due to high temperature for heat preservation time, the saturated C in crystal silicon has been nucleated and grown. It may also the reasons for the appearance of SiC inclusions in mc-Si ingots for solar cells. But the reasons for the SiC inclusions not subsiding in melting silicon need further study.