Rapid Assessment of As-grown Interstitial Iron Concentration and Impact on P- Type Silicon Blocks via Spectral Photoluminescence Imaging

Early stage material quality control directly after crystallization is becoming increasingly sophisticated. It is enabled through the characterization of various material parameters along the silicon ingot/brick side faces. Photoluminescence techniques can provide quantitative measures for example of the important photovoltaic material parameter of bulk lifetime. It can measure this parameter with full spatial resolution within a few seconds. Here, we demonstrate the extension of the spectral photoluminescence analysis on silicon bricks to the extraction of the dissolved iron concentration. The analysis uses 1) the reversible pairing that interstitial iron undergoes with the acceptor atom at room temperature in the dark after iron boron pairs were dissociated with an intense flashlight and 2) the respective lifetime fingerprint of these fairly well known defects. We can perform the analysis without any extra sample preparation on the bare brick, measure at true low injection conditions and are able to integrate it into a production environment. We demonstrate the analysis on a widely used 6 inch multicrystalline silicon brick that was directionally solidified from electronic grade feedstock. With access to both bulk lifetime and interstitial iron concentration, the relative impact of the interstitial iron concentration can be quantified and a theoretical bulk lifetime after removal of all interstitial iron contamination can be calculated. Both of these represent interesting early and direct measures relevant to the further understanding of the crystallization and following cell processing. Interstitial iron has been found to contribute majorly to the overall recombination all through the as-grown ingot and is dominating the recombination in the highly contaminated and low lifetime bottom of the ingot. The combined access to spatial information of both bulk lifetime and interstitial iron concentration is found to be a valuable tool to also assess the contribution of other defect contaminations, its distributions and, with some additional information aside, possibly also its origins and causes.