Modifications of Textured Silicon Surface Morphology and its Effect on Poly-Si/SiOx Contact Passivation for Silicon Solar Cells

High-efficiency crystalline silicon (c-Si) solar cells require textured surfaces for efficient light trapping. However, passivation of a textured pyramidal surface to reduce carrier recombination is challenging due to the presence of sharp tips, edges, and valleys. Using electrical and surface microscopies combined with lifetime measurements, we report on the effect of HF:HNO₃ etching on the pyramidal textured Si surface morphology, and on poly-Si/SiO_x contact performance. Preferential rounding of either the valleys between pyramids, or the pyramid tips is obtained depending on the HF:HNO₃ solution temperature. Both these morphologies make the pyramid shape irregular, with the pyramid faces no longer being predominantly a Si(111) surface. Our atomic force microscopy measurements further show that the nanoscale roughness over the pyramid face also reduces after HF:HNO₃ etching. Thus, etching affects the microscopic pyramidal shape, the dominant crystallographic orientation, and the nanoscale roughness of the surface. We speculate that these three surface effects result in the improvement of surface passivation via poly-Si/SiO_x contacts. However, this improved passivation is accompanied by increased reflectance of the HF:HNO₃-etched textured surface. Finally, our electron-beam-induced current measurements reveal thickness nonuniformities in the thermally grown SiO_x layer on the HF:HNO₃-etched textured Si surface: SiO_x is thicker near the pyramid tips, edges, and faces as compared with near the valleys between pyramids. This nonuniformity in the SiO_x layer may explain the poorer passivation obtained on a KOH-textured and HF:HNO₃-etched textured surface as compared with a planar surface.