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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 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 (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 mu 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

Instrumental neutron activation analysis was performed to determine the transition metal content in three types of silicon material for cost-efficient solar cells: Astropower silicon-film sheet material, Baysix cast material, and edge-defined film-fed growth (EFG) multicrystalline silicon ribbon. The dominant metal impurities were found to be Fe (6x10(14) cm(-3) to 1.5x10(16) cm(-3), depending on the material), Ni (up to 1.8x10(15) cm(-3)), Co (1.7x10(12) cm(-3) to 9.7x10(13) cm(-3)), Mo (6.4x10(12) cm(-3) to 4.6x10(13) cm(-3)), and Cr (1.7x10(12) cm(-3) to 1.8x10(15) cm(-3)). Copper was also detected (less than 2.4x10(14) cm(-3)), but its concentration could not be accurately determined because of a very short decay time of the corresponding radioactive isotope. In all samples, the metal contamination level would be sufficient to degrade the minority carrier diffusion length to less than a micron, if all metals were in an interstitial or substitutional state. This is a much lower value than the actual measured diffusion length of these samples. Therefore, most likely, the metals either formed clusters or precipitates with relatively low recombination activity or are very inhomogeneously distributed within the samples. No significant difference was observed between the metal content of the high and low lifetime areas of each material. X-ray microprobe fluorescence spectrometry mapping of Astropower mc-Si samples confirmed that transition metals formed agglomerates both at grain boundaries and within the grains. It is concluded that the impact of metals on solar cell efficiency is determined not only by the total metal concentration, but also by the distribution of metals within the grains and the chemical composition of the clusters formed by the metals

Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cells J. Appl. Phys. 97, 074901 (2005); 10.1063/1.1866489 Metal content of multicrystalline silicon for solar cells and its impact on minority carrier diffusion length J. Appl. Phys. 94, 6552 (2003); 10.1063/1.1618912Nanometer-scale metal precipitates in multicrystalline silicon solar cells

A synchrotron radiation based x-ray microprobe analytical technique, x-ray beam induced current ͑XBIC͒, is suggested and demonstrated at the Advanced Light Source at the Lawrence Berkeley National Laboratory. The principle of XBIC is similar to that of electron/laser beam induced current with the difference that minority carriers are generated by a focused x-ray beam. XBIC can be combined with any other x-ray microprobe tool, such as the x-ray fluorescence microprobe ͑-XRF͒, to complement chemical information with data on the recombination activity of impurities and defects. Since the XBIC signal, which carries information about the recombination activity of defects in the sample, and the-XRF signal, which contains data on their chemical nature, can be collected simultaneously, this combination offers a unique analytical capability of in situ analysis of the recombination activity of defects and their chemical origin with a high sensitivity and a micron-scale spatial resolution. Examples of an application of this technique to multicrystalline silicon for solar cells are demonstrated.