Solar Grade Silicon (S. Binetti, M. Acciarri)

It is widely known that PV industry is expected to remain mostly based on crystalline silicon at least for the next decade, therefore a substantial increase  in the Si production is needed both in order to sustain the high PV growth rate and to decrease the cost per Watt peak. For this aim, efforts were made in order to develop processes for the purification and the use of metallurgical grade Si, which results in low cost but less pure Si, the so-called solar grade silicon (SoG-Si).
In this frame our research activities is devoted to the study of the effect of the impurities on the lifetime and efficiency of the up grade metallurgical silicon based solar cell.
Besides a variety of metallic impurities, SoG-Si often contains a large amount of the doping elements. Thereby, the effect of the compensation, has to be taken into account, since this can impact solar cell performance. In our research activities, the spectroscopic and electrical features of compensated Si samples has been monitored by different techniques (Hall Effect , Photoluminescence and Infrared spectroscopy). Futhermore considering the multicrystalline nature of the silicon used, the recombination activity of the exdended defects and its dependence with the impurities content should also be monitored. While grain boundaries in clean samples show very low recombination activity almost independent on misorientation and temperature, this is not the case for dirty samples.
In our studies the electrical activities of extended defects (i.e grain boundaries and dislocations in mc-Si) grown from metallurgical feedstock, have been investigated in detail using electron beam induced current (EBIC) and photoluminescence spectroscopy.  The EBIC analysis is systematically used to identify the electrical active extended defects in the material, such as grain boundaries and dislocations. Finally also the role of the oxygen distribution on a multicrystalline silicon ingot produced by directional solidification of upgraded metallurgical silicon is also investigated by lifetime measurements, infrared spectroscopy (also at low T) in the as grown and properly annealed samples.

Post-doctoral fellow: Alessia Le Donne

Selected Publications

This Figure shows an EBIC map in (a), and EBSD map in (b). The arrow point out a possible small angle grain boundary with very high recombination activity observed in EBIC, not observed in the EBSD map where the detection limit for grain boundaries are set to 2°. A TEM analysis of the same area has shown a large dislocation heavily decorated with silicon oxide precipitates.