Origin of the Low Carrier Lifetime Edge Zone in Multicrystalline PV Silicon

An investigation of impurities, crystal defects and microstructure has been performed on the edge zone, i.e. close to the crucible wall, which experiences reduced carrier lifetime in a directionally solidified multicrystalline p-doped silicon ingot. The characterization methods applied have been QSSPC, FTIR, mu W-PCD, EBSD, CDI, PVScan, optical microscopy, FeB-pair splitting and GDMS. The results of the minority carrier lifetime measurements have revealed strong v reduced values in the vicinity of the edge (< 1 mu s). Increased values were obtained starting at 1.5-17 mm from the edge. Light elements analyses showed that the O, N and C concentrations, interstitially, or in particles, did not increase in the edge zone, neither did the dislocation density. GDMS analyses detected traces of aluminium, iron, copper, titanium and chromium. The total iron concentration showed an increase towards the edge, though high concentrations were occasionally detected in the bulk Fell pair analysis revealed large concentrations of Fe (similar to 1 X 10(13) cm(-3)) in the vicinity of the edge with a distinctively decreasing trend moving away front the edge. The detected Fell-concentrations are sufficient to account for the majority of the 1 lifetime degradation close to the edge (0-15 mm). In addition, Fe, in the form of Fell pairs, was extensively observed as object to internal gettering to high angle boundaries and dislocations. Fe, in the form of FeB pairs, is furthermore believed to originale from solid state diffusion from the crucible and coating. Copyright (C) 2008 John Wiley & Sons, Ltd.