Minority carrier bulk lifetimes through a large multicrystalline silicon ingot and related solar cell properties

The bulk lifetime $\tau _{n}$ and diffusion length Ln of minority carriers vary through the height of a cast multicrystalline silicon (mc-Si) block. This variation is due to the segregation of metallic impurities during the directional solidification and the native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible floor. It is of interest to verify if a correlation exists between the bulk lifetime $\tau $ of as cut wafers and the conversion efficiency $\eta $ of solar cells. In a very large ingot (>310 kg), it was found that $\tau_{0}$, in raw wafers, $\tau_{dif}$ in phosphorus diffused ones and Ln in diffused wafers are smaller in the top and in the bottom of the ingot. The same evolution is observed in solar cells, however the diffusion length values Lcel in the central part of the ingot are markedly higher than those found in diffused wafers, due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations of $\eta $ and those of $\tau _{0}$, along the ingot height, are well correlated, suggesting that the evaluation of $\tau _{0}$ can predict the properties of the devices. In addition, segregation phenomena around the grain boundaries are observed at the bottom of the ingots, due to a marked contamination by the crucible floor, and at its top where impurities are accumulated. These phenomena are linked to the long duration of the solidification process and the large amount of imperfect silicon used to cast the ingot