Protection of Si-SiO2 interfaces from damp heat by overlying SiNx and Si3N4 coatings

PECVD SiN_x antireflection coatings are found to suppress the degradation of an underlying oxide during “damp-heat” exposure. Samples are submitted to 85°C and 85% relative humidity, replicating the damp-heat conditions of the standard reliability tests for PV modules. The samples are diffused with phosphorus and passivated with a thermal oxide, where the resistivity of the diffusion and the oxide thickness are similar to those used in many high-efficiency silicon solar cells. We find that when the samples are not coated with SiN_x, exposure to 1000 hours of damp-heat causes their emitter saturation current density J_0E to increase from 32 to 53 fA/cm² on planar (100) wafers, and from 42 to 95 fA/cm² on textured wafers. We show that this degradation requires the presence of water vapour and does not occur due to the elevated temperature alone. These results suggest that high-efficiency silicon cells would fail the damp-heat reliability test if their SiO₂ were not protected from the damp-heat. Happily, the degradation is strongly suppressed when the samples are coated with a PECVD SiN_x antireflection coating. This suggests that the SiN_x limits the diffusion of water vapour into the SiO₂. We also find that LPCVD Si₃N₄ suppresses damp-heat degradation although the results are less conclusive due to the initial J_0E being many times higher than for PECVD SiN_x.