Gettering of metallurgical grade silicon by HCl gas
In this thesis, hydrogen chloride (HCl) gas gettering of multicrystalline silicon for the use in photovoltaics is investigated. The gettering effect is due to the removal of metallic impurities from the surface of the wafers through the formation of volatile chloride compounds. This method is considered to be used as an effective purification step in the solar cell process. It could be easily integrated into the concept of the crystalline silicon thin film (cSiTF) solar cell. This concept combines two main aspects. On the one hand, it is cost saving because only a small amount of expensive high purity silicon is needed for the epitaxial deposition of a thin layer on low-cost silicon. On the other hand, the product which is called epitaxial wafer equivalent (EpiWE) can be further processed in a standard wafer cell production. However, low-cost silicon which could be used as substrate has a high amount of doping and metallic impurities which severely degrades the cell efficiency. Therefore, the amount of impurities in the substrate has to be reduced in order to prevent diffusion into the active layer. An economical and effective purification step is needed. HCl gas gettering shows the potential to achieve the required purity levels.