Metal Content of Multicrystalline Silicon for Solar Cells and its Impact on Minority Carrier Diffusion Length

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 (6×1014 cm−3 to 1.5×1016 cm−3, depending on the material), Ni (up to 1.8×1015 cm−3), Co (1.7×1012 cm−3 to 9.7×1013 cm−3), Mo (6.4×1012 cm−3 to 4.6×1013 cm−3), and Cr (1.7×1012 cm−3 to 1.8×1015 cm−3). Copper was also detected (less than 2.4×1014 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...