Long-term stability study of the passivation quality of polysilicon-based passivation layers for silicon solar cells

Abstract

We investigate the stability of the effective lifetime τeff and the recombination current density parameter J0 in ntype silicon samples with symmetric phosphorus doped poly-Si/SiOx structures, and identify factors that contribute to the passivation degradation behavior. It is found that the surface passivation quality of phosphorus doped polysilicon passivating contacts degrades upon dark annealing and light soaking at temperatures between 75 °C and 200 °C, which can lead to a pronounced increase of the recombination current density parameter J0 (one-side) from below 10 fA/cm2 to 50 fA/cm2 or above. The degradation is only detected on fired wafers, whereas the surface passivation quality is found to be stable in the non-fired sister samples. Surprisingly, a recovery of τeff and J0 is observed after the degradation. The degradation and regeneration behaviors depend strongly on temperature and light intensity, and the presence of silicon nitride (SiNx) capping layers during the light soaking. Increasing the annealing temperature dramatically increases the rate of the degradation and the regeneration process, and at the same time reduces the magnitude of the degradation. The regeneration process appears to be affected by the presence of SiNx films during the light soaking treatment. Samples with SiNx films removed after firing suffer a significantly larger degradation upon light soaking without any lifetime regeneration. Grazing incidence X-ray diffraction measurements reveal negligible change in the structural property and crystalline quality of the polysilicon layer during the degradation.