Structural and electrical charaterization of crystallographic defects in silicon ribbons

Abstract

Silicon ribbons grown as wide as 50 mm by the Capillary Action Shaping Technique (CAST) are classified according to perfection and electrical performance. The classification is obtained through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and minority carrier lifetime and solar cell efficiency measurements. First the electrical activity of the defects is measured through EBIC contrast in the SEM. Subsequently the defects are analyzed in the TEM. The defect state in the crystal thus obtained is correlated with minority carrier lifetime and related to solar cell efficiency. Four crystal quality groups are established for CAST ribbons. In these groups solar cell efficiency decrease from 5–8% in class I to less that 1% in class IV. Such degradation in solar cell efficiency is caused by dislocations (I) high order twin boundaries (II), low angle grain boundaries (III), and, silicon carbide dendrites (IV). The electrical activity of such defects varies from low activity to strong activity. Solar cell efficiency depends on the number of “electrically active defects” and not on the total defect content of the ribbon.