We present a comprehensive summary of our observations of metal-rich particles in multicrystalline silicon (mc-Si) solar cell materials from multiple vendors, including directionally-solidified ingot-grown, sheet, and ribbon, as well as multicrystalline float zone materials contaminated during growth. In each material, the elemental nature, chemical states, and distributions of metal-rich particles are assessed by synchrotron-based analytical x-ray microprobe techniques. Certain universal physical principles appear to govern the behavior of metals in nearly all materials: (a) Two types of metal-rich particles can be observed (metal silicide nanoprecipitates and metal-rich inclusions up to tens of microns in size, frequently oxidized), (b) spatial distributions of individual elements strongly depend on their solubility and diffusivity, and (c) strong interactions exist between metals and certain types of structural defects. Differences in the distribution and elemental nature of metal contamination between different mc-Si materials can largely be explained by variations in crystal

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

Scanning photoluminescence (PL) spectroscopy was performed on as-grown and processed multicrystalline silicon (mc-Si) wafers to investigate the defect distribution affecting the efficiency of solar cells. In highly inhomogeneous mc-Si prepared by (i) edge-defined film-fed growth or (ii) a block-casting technique, regions of a wafer with enhanced recombination activity and reduced minority carrier lifetime exhibit an intensive 'defect' PL band at room temperature with the maximum at about 0.8 eV. By comparing PL mapping with the distribution of dislocations, we present experimental evidence that the 0.8 eV band corresponds to electrically active dislocation networks. This was confirmed using low-temperature PL spectroscopy, which revealed a characteristic quartet of the dislocation D-lines. One of these dislocation lines, D1, can be tracked as temperature increases and linked to the 'defect' band. Strong linear polarization of the 0.8 eV PL band corresponds to a preferential localization of defects in regions with a high level of elastic stress measured with scanning infrared polariscopy. The origin of the 0.8 eV PL band is attributed to dislocations contaminated with impurity precipitates.

A promising method to introduce H into multicrystalline Si solar cells in order to passivate bulk defects is by the postdeposition annealing of a H-rich, SiN x surface layer. It has previously been difficult to characterize the small concentration of H that is introduced by this method. Infrared spectroscopy has been used together with marker impurities in the Si to determine the concentration and depth of H introduced into Si from an annealed SiN x film.

The metal content of three types of silicon material for cost-efficient solar cells, Astropower silicon-film sheet material, Baysix cast material, and EFG ribbon-grown multicrystalline silicon was determined using instrumental neutron activation analysis. Iron, nickel, and chromium were found in concentrations between 10 12 and 1.8´10 15 cm -3 , depending on the material. The concentration of cobalt, molybdenum, and copper was between 10 12 and 5´10 13 cm -3 . Since the minority carrier diffusion length in all three mc-Si materials was much higher than one would expect if all metals were dissolved in an interstitial or substitutional state, we concluded that the metals most likely formed clusters or precipitates with a lower recombination activity than the interstitial/substitutional metals. No significant difference was observed between the metal content of the high and low lifetime areas of each material.