Investigation of Defects in the Edge Region of Multicrystalline Solar Silicon Ingots

Abstract

The structural and electrical properties of as grown multicrystalline (mc) solar silicon have been characterized with special emphasis on the ingot's edge regions. For this purpose a vertical cross section of an mc-Si Bridgman ingot was investigated by Fourier transform infrared spectroscopy (FTIR), laser scattering tomography (LST), lateral photovoltage scanning (LPS), infrared microscopy and microwave detected photoconductivity decay (&PCD). Images of the distribution of dislocations, grain boundaries, precipitates, impurities (O, N, C) and the minority charge carrier lifetime were obtained, partly differentiating the defects by their electrical activity. In particular the LPS method displays dopant striations indicating the shape of the phase boundary. Deviations of the phase boundary from a slightly convex shape in the middle of the ingot to a concave one in the vicinity of the side walls could be observed. The existence of an horizontal temperature gradient deduced from this shape is the reason for convection in the melt. The influence on the concentration profiles of interstitial oxygen and the correlation with the minority charge carrier lifetime are discussed.