Characterization of high-efficiency cast-Si solar cell wafers by MBIC measurement

Cast-Si wafers for high-efficiency solar cells were characterized by MBIC measurement, in which the minority-carrier recombination loss at structural defects were seen as a drop in intensity. Cast-Si wafers contained a large number of non-planer and irregular grain boundaries, the majority if which had the electrical activity greatly due to a slight difference in the crystal orientation. These boundaries were classified into three geometrical structures, that is, columnar-structure grain boundary (GB), tilt-structure GB and GB structure with a transition interface layer. Other dominant defects were planar boundaries, {111} twin boundaries, {221} second-order twin boundaries and {221} planar boundaries without a twinning relationship (symmetric twist boundaries) which were found to be electrically inactive and were mainly observed in the high-quality wafers. Besides, the boundary structures off the twinning relationship, fine-crystalline structures and subgrain boundary structures were found to have remarkable electrical activity.