We report on progress with PL imaging applications in silicon solar cell production, specifically focusing on the characterization of silicon bricks prior to wafer cutting. Silicon bricks represent an ideal opportunity to characterize and quantify the electronic material quality at an early stage of the PV value chain. Quantitative data on bulk lifetime can be obtained on bricks without any specific sample preparation, unlike unprocessed wafers. Spatially resolved bulk lifetime, interstitial iron concentration, and defect density measurements are demonstrated on bricks from different manufacturers including both high performance multicrystalline and older generation multicrystalline bricks. We find significant variability in bulk lifetime and iron concentration across the samples which is not related to its date of manufacture. However we do see a qualitative reduction in crystallographic defects in the newer high performance multicrystalline bricks. Data is parameterized in different ways to suggest possible paths to better predict solar cell efficiencies from an early stage of inspection.
Brick level PL measurements were previously performed using a conventional area scanning PL imaging system, which is associated with light spreading artefacts of weakly absorbed light. To overcome these artefacts, a new line scanning photoluminescence imaging system is used. We show a reduction in contrast smearing between high- and low lifetime regions in the new setup leading to image quality suitable for defect detection and quantitative measurements without deconvolution correction.