Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cells

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Abstract
Synchrotron-based microprobe techniques were used to obtain systematic information about the size distribution, spatial distribution, shape, electrical activity, chemical states, and origins of iron-rich impurity clusters in multicrystalline silicon (mc-Si) materials used for cost-effective solar cells. Two distinct groups of iron-rich cluster have been identified in both materials: (a) the occasional large (diameter ⩾ 1 μ 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 (diameter ⩽ 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 10 14 – 10 15 cm − 3 and can have a large negative impact on device performance because of their high spatial density and homogeneous distribution along structural defects. The large (diameter ⩾ 1 μ m ) particles, while containing elevated amounts—if not the majority—of metals, are low in spatial density and thus deemed to have a low direct impact on cell 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 the mc-Si device performance but the distribution of Fe within the material.