Will SiOx-pinholes for SiOx/poly-Si passivating contact enhance the passivation quality?

Passivating contacts based on poly-Si have enabled record-high c-Si solar cell efficiencies due to their excellent surface passivation quality and carrier selectivity. The eventual existence of pinholes within the ultra-thin SiO x layer is one of the key factors for carrier collection, beside the tunneling mechanism. However, pinholes are usually believed to have negative impact on the passivation quality of poly-Si passivating contacts. This work studied the influence of the pinhole density on the passivation quality of ion-implanted poly-Si passivating contacts by decoupling the pinhole generation from the dopants diffusion process by means of two annealing steps: (1) a pre -annealing step at high temperature after the intrinsic poly-Si deposition to visualize the formation of pinholes and (2) a post -annealing step for dopants activation/diffusion after ion-implantation. The pinhole density is quantified in the range of 1✕106 to 3✕108 cm2 by the TMAH selective etching approach. The passivation quality is discussed with respect to the pinhole density and the post-annealing thermal budget (TB) for dopants diffusion. The study shows that a moderate pinhole density does not induce doping profile variations that can be detectable by the coarse spatial resolution of ECV measurements. It is surprising that the existence of pinholes in a moderate density within our thickness fixed SiO x layer can effectively enhance the passivation qualities for both n + and p + poly-Si passivating contacts. We speculate the reason is due to the enhanced field-effect passivation at the pinhole surrounding. In fact, the variation of the passivation quality depends on the balance between a strengthened field-effect passivation and an excessive local Auger recombination, being both effects induced by the higher and deeper level of dopants diffused into the c-Si surface through the pinholes. • Decoupling of the pinhole formation and the dopant diffusion using two annealing processes. • Application of the thermal diffusion budget concept to the fabrication of the poly-Si passivating contacts. • The existence of pinholes within the SiO x layer enhances the localized dopant diffusion. • The dopant diffusion through the pinholes enhances the electrical field passivation locally. • A moderate pinhole density within SiO x induces obvious increase in passivation qualities for both n+ and p+ poly-Si passivating contacts, although the sensitivity of passivation on pinhole density is less for p + poly-Si. [ABSTRACT FROM AUTHOR]