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Micro-cracks in silicon wafers and solar cells: Detection and rating of mechanical strength and electrical quality
In this work, we summarize the basic results of two studies investigating the detection of micro-cracks in as-cut wafers, their impact on fracture strength after texturing (criterion 1) and finally the electrical quality after solar cell production (criterion 2). As a key parameter for quality assessment, the morphology of the artificially induced cracks is reconstructed and quantified in photoluminescence (PL) images of as-cut wafers. As expected, the crack-related sorting parameter showed high correlation to both quality parameters. This enables us to define classification criteria to rate the measured crack morphology in the as-cut stage in terms of the expected fracture strength of the wafer and the expected parallel resistance of the cell, respectively. Based on the presented studies, sorting criteria are evaluated via a receiver operating characteristic (ROC) analysis assuming a specific production load. Moreover, requirements for a PL detection algorithm are derived to achieve the optimal sorting quality according to specific limits for fracture strength and parallel resistance. Cost of Ownership calculations show, that sorting out as-cut wafers prior to cell production can be beneficial if the crack detection system's sorting quality is sufficient.