Influence of Copper Diffusion on Lifetime Degradation in n-type Czochralski Silicon for Solar Cells

The impact of copper (Cu) contamination on the minority carrier lifetime degradation of the last solidified fraction of ann-type Czochralski (CZ) silicon ingot was investigated. Lifetime degradation of more than 50% was measured at the top of an as-cut block during the first 2000 h (∼83 days). Depth profiles of Cu concentration were collected over a depth of 20 μm from the surface and an exponential decrease with depth was observed. The critical concentration for Cu precipitation was exceeded within the top 2 μm. The mechanism of Cu diffusion and precipitation towards the surface at room temperature is the suggested phenomenon for the lifetime degradation. The effect of surface potential on Cu gettering was investigated in neighboring samples by grinding the surface with different SiC grit sizepri or to storage in the dark. The measured Cu profiles show that an increased surface area enhances the Cu precipitation at the surface and leads to a lower concentration deeper in the bulk. Since Cu diffuses over large distance at room temperature in n-type CZ silicon and it is mainly segregated at the last solidified part of the ingot, we suggest that the tail should be detached from the main body of the ingot in order to remove a potential source of Cu contamination.