Metal content of multicrystalline silicon for solar cells and its impact on minority carrier diffusion length

Abstract

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 ${\text{(6×10}}^{14}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ to ${\text{1.5×10}}^{16}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}},$ depending on the material), Ni (up to ${\text{1.8×10}}^{15}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}\mathrm{),}$ Co ${\text{(1.7×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ to ${\text{9.7×10}}^{13}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}\mathrm{),}$ Mo ${\text{(6.4×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ to ${\text{4.6×10}}^{13}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}\mathrm{),}$ and Cr ${\text{(1.7×10}}^{12}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ to ${\text{1.8×10}}^{15}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}\mathrm{).}$ Copper was also detected (less than ${\text{2.4×10}}^{14}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}\mathrm{),}$ 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.