Nonmetallic Inclusions in Solar Cell Silicon: Focusing on Recycling of Scraps

The solar grade silicon ingot produced from directional solidification process usually pushes the impurities to the top and finally cut off and discarded, which leads to material loss. The hard inclusions lead to wire breakages during the cutting of the ingot in wafers. The main kinds of inclusions found in solar grade silicon have been investigated using vacuum filtration: needle-like Si3N4 and lumpy SiC inclusions. Clusters of SiC inclusions and Si3N4 are also found. Surface observations of the scraps before polishing reveals that Si 3 N 4 inclusions are usually bigger and in some cases can be about a few millimeters. SiC inclusions are usually smaller, ~200mum but can be ~500mum in some cases. Inclusions observed after filtration are mainly SiC with diameters ~10mum. Through the vacuum filtration, 99% inclusions can be removed 99%. The possible mechanisms of filtration are cake filtration for the larger sized Si 3 N 4 and SiC inclusions and deep-bed filtration the SiC inclusions. For the directional solidified silicon ingot, an approximate distance of ~10mm gave an encouraging cutoff thickness. The inclusions sizes were below 10 m.