Response to phosphorus gettering of different regions of cast multicrystalline silicon ingots

Abstract

Minority carrier lifetimes were measured to determine the effect of phosphorus gettering on cast multicrystalline silicon solar cell substrates from central and end regions of two different ingots. One ingot exhibited visibly inferior crystallographic structure and, correspondingly, showed lower lifetimes. Wafers from the bottom region of both ingots improved significantly after gettering, whilst those from the top experienced no lifetime increase. Defect etching revealed that the wafers from the top had very high dislocation densities (> 106 cm-2), whereas wafers from the bottom had low dislocation densities (104 cm-2). Novel cross-contamination experiments showed that all top and bottom wafers contained high concentrations of mobile (and hence getterable) impurities in comparison to central regions, irrespective of dislocation density. For the case of the highly dislocated samples, a sufficient number of recombination centers remains to prevent an increase in the lifetime, even after the mobile impurities have been extracted through gettering. Wafers from central regions had both moderate dislocation densities and low concentrations of out-diffusible impurities. Their lifetimes increased after gettering up to a more evenly distributed limit imposed by the crystallography, which was found to be ingot and region dependent.