Reducing the impact of metal impurities in block-cast mc Silicon

Abstract

The detrimental influence of transition metals on minority carrier lifetime in solar cells and thus solar cell performance is up to now not fully understood. In this paper we would like to contribute some new results on this topic concerning the gettering and deactivation of the transition metals iron (Fe) and copper (Cu) in block cast multicrystalline (mc) Si material. The investigated processing steps include extended POCl3 gettering and hydrogenation by firing of a PECVD (Plasma-Enhanced Chemical Vapor Deposition) silicon nitride (SiNx:H). They are carried out on neighboring wafers using processing conditions which are derived from the standard photolithography based process at the University of Konstanz (UKN). Sets of neighboring wafers are selected from bottom, middle and top of an ingot, respectively, to address the varying impurity concentrations e.g. due to segregation during ingot casting. To extract the specific influence of the transition metals, wafers from intentionally contaminated mc material were examined. Hydrogenation clearly turned out to have the biggest positive influence on the contaminated material, while extended gettering showed non-uniform results. In some cases an improvement was observed, but also degradation of the bulk material occurred.