Grain-to-grain contrasts in photoluminescence images of silicon wafers

Photoluminescence (PL) imaging of silicon wafers has become a very valuable characterization technique over the last decade since it is fast, non-destructive and can be applied to finished cells as well as cell precursors. In this contribution, we examine the grain-to-grain contrasts observed in PL images of as-cut, mechanically polished and alkaline textured wafers from multicrystalline (mc) material and material with mono seeds. Understanding the contrasts in PL images of as-cut wafers is of special importance, because PL images of as-cut wafers are used for quality rating and the prediction of final solar cell efficiency. In some cases, grain-to-grain contrasts can dominate the image appearance while their origin remains unclear. Therefore, we investigate the reasons for the observed grain-to-grain contrasts in mc silicon and material with mono seeds. In the as-cut state, the reason for grain-to-grain contrasts is found to be an interplay between different reflectivity of the excitation light and different electrical surface properties. In the mechanically polished state, there are no optical differences between grains and the differences of PL intensity contrasts solely originate from different electrical surface properties whereas for alkaline textured surfaces optical effects are the dominant reason for PL intensity contrasts.