Lawrence Berkeley National Laboratory

Synchrotron-based microprobe techniques were used to obtain precise and systematic information about the size distribution, spatial distribution, shape, electrical activity, and chemical states of iron-rich impurity clusters in multicrystalline silicon materials used for cost-effective solar cells. These experimentally observed properties of iron-rich clusters allow one to derive conclusions about the origins of iron contamination, the mechanisms for incorporating large amounts of Fe into mc-Si, quantitative information about the distribution of Fe in mc-Si and the impacts of such contamination on solar cell performance. Two distinct groups of iron-rich clusters have been identified in both materials: (a) the occasional large (diameter greater than or equal to 1 mu-m) particles, either oxidized and/or present with multiple other metal species reminiscent of stainless steels or ceramics, which are believed to originate from a foreign source such as the growth surfaces, production equipment, or feedstock, and (b) the more numerous, homogeneously distributed, and smaller iron silicide precipitates (dia. less than or equal to 800 nm, often < 100 nm), originating from a variety of possible formation mechanisms involving atomically dissolved iron in the melt or in the crystal. It was found that iron silicide nanoprecipitates account for bulk Fe concentrations as high as 1014-15 cm-3 and can have a large negative impact on device performance because of their homogeneous distribution along structural defects. The large (dia. greater than or equal to 1 mu-m) particles, while containing elevated amounts of metals, are low in spatial density and thus deemed to have a low direct impact on device performance, although they may have a large indirect impact via the dissolution of Fe, thus assisting the formation of iron silicide nanoprecipitates. These results demonstrate that it is not necessarily the total Fe content that limits mc-Si device performance, but the distribution of Fe within the material.