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:HNO
₃
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:HNO
₃
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:HNO
₃
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:HNO
₃
-etched textured surface. Finally, our electron-beam-induced current measurements reveal thickness nonuniformities in the thermally grown SiO
_x
layer on the HF:HNO
₃
-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:HNO
₃
-etched textured surface as compared with a planar surface.