Rating and sorting of mc-Si as-cut wafers in solar cell production using PL imaging

Photoluminescence (PL) imaging is a promising characterization technique for rating and sorting of multicrystalline silicon (mc-Si) as-cut wafers concerning to their material quality. It is inline applicable and yields high resolution images showing recombination active defects from crystallization which influence solar cell performance. In this contribution the basic concepts in ongoing work concerning relevant defects, rating results, statistics, algorithms and general approaches for the rating are summarized. Since 2009 wafers are sorted into five quality classes at Fraunhofer ISE. Details on the rating criteria are given and three examples for application are presented: (i) the impact of edge contaminations on the solar cell results, (ii) a comparison of wafer suppliers based on random samples from a statistical basis of 10,000 wafers and (iii) the results of an advanced rating and sorting of wafers for the manufacturing of highly efficient MWT-PERC solar cells. The results confirm that PL imaging can be used for a very precise rating of material quality.