The electrical properties and the minority charge carrier recombination behaviour of grain boundaries and intragrain dislocations in various n-type multicrystalline Si (mc-Si) ingots were systematically studied via microwave-detected photoconductance decay, Electron beam induced current and photoluminescence spectroscopy on as-grown samples and on samples submitted to P-diffusion step. It was confirmed that the overall quality of n-type mc-Si was high, indicating that n-type-Si was a valid source for photovoltaic applications. As expected, the average lifetime increased after the P-diffusion process, which induces impurity gettering effects at the external surfaces, like in the case of p-type samples, but an evident local increase of electrical activity of some grain boundaries after that process was also observed using the electron beam induced current mapping technique. Apparently, a redistribution of impurities occurred at the processing temperature and impurities were captured at the deepest sinks. In fact, while all grain boundaries act as heterogeneous segregation/precipitation sites, some of them will compete with the external surfaces sinks, partly vanishing the effect of P-gettering. Last but not least, it was experimentally demonstrated that the average lifetime values measured with the microwave-detected photoconductance decay technique well correlated with the recombination activity of grain boundaries measured with the electron beam induced current technique, showing the extreme importance of grain boundaries on the effective lifetime of this material.

Effect of P-Induced Gettering on Extended Defects in n-Type Multicrystalline Silicon. M.Acciarri, S.Binetti, A.Le Donne, S.Marchionna, M.Vimercati, J.Libal, R.Kopecek, K.Wambach: Progress in Photovoltaics, 2007, 15[5], 375-86