Classification of defective regions in p-type multicrystalline silicon by comparing luminescence images measured under different conditions

Abstract

In this contribution, we apply three different camera-based luminescence imaging techniques to mc-Si wafers and solar cells, fabricated on neighboring wafers. On wafer level, we determine the spatially-resolved carrier lifetime using calibrated photoluminescence lifetime imaging. On the solar cell level, we use band-to-band electroluminescence and sub-band-gap electroluminescence imaging for the characterisation. We analyze the differences obtained by the different techniques in specific defective areas. Characteristic regions are additionally examined using deep-level transient spectroscopy (DLTS). Comparing different luminescence images, we find different signal correlations in selected regions of the wafers and the neighboring cells presumably caused by different types of defects, which react more or less effective on the phosphorus gettering during the solar cell process. DLTS spectra show that in the edge region of the wafer close to the crucible, FeB pairs are present in the wafer as well as in the cell. However, the FeB concentration in the cell is, due to phosphorus gettering during the cell process, reduced by one order of magnitude. In regions which appear as very recombination-active defect clusters in the solar cell, we detect ZnB pairs by DLTS analysis. Note that the ZnB itself is a shallow centre and therefore expected to be not strong recombination active. However, our measurements reveal that Zn is present in regions with increased recombination activity, which is also in good agreement with the high total Zn concentration measured in the mc-Si ingot. We hence conjecture that dislocation clusters decorated by Zn are responsible for the non-getterable defect regions.