A study of mottling phenomenon on textured multicrystalline silicon wafers and its potential effects on solar cell performance

The mottling phenomenon refers to the appearance of irregular dark patterns on HF–HNO₃ acid etching textured multicrystalline silicon (mc-Si) wafers. The mottles have been identified as clusters of dislocation etch pits. In the acidic texturization, conditions favoring light reflectivity reduction usually lead to enhanced mottling. In a belief of adverse effect of the mottles to cell performance, light reflectivity reduction is more or less compromised in industry to avoid the mottling. The present study aims to identify whether appearance of the mottles alone really adversely affects the wafers minority carrier lifetime. Both serial examinations of an acid etched mc-Si wafer sample and parallel examinations of neighboring pairs of mc-Si wafer samples etched in acids of different HF/HNO₃ ratios were carried out. The results show that development of the mottling, i.e., growth of dislocation etch pits, does not deteriorate mc-Si wafers in their minority carriers lifetime; rather it even slightly increases the lifetimes. Light reflectivity measurement and modeling show that the mottles can contribute to reduction of light reflectivity, and ~3% relative reduction of reflectivity is expected for multicrystalline silicon wafers of ordinary level of dislocation density. Removal of the defected zone surrounding a dislocation by the etching is postulated as a reason for the observed mottling-enhancement of the lifetime. It is further postulated that, in texturization of mc-Si wafers for cell production, instead of compromising light reflectivity reduction to avoid the mottling, it may be better to pursue lower light reflectivity, allowing some extent of the mottling. Meanwhile, more attention should be paid to compatibility of the cone-shaped dislocation etch-pit with grid printing of solar cells.