Protection of Si-Sio₂ interfaces from damp heat by overlying SiN_x 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 highefficiency 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_OE to increase from 32 to 53 fA/cm² on planar (100) wafers, and from 42 to 95 fA/cm ²on 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 highefficiency 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_xantireflection coating. This suggests that the SiN_xlimits the diffusion of water vapour into the SiO ₂. We also find that LPCVD Si₃N₄ suppresses damp-heat degradation although the results are less conclusive due to the initial J_OE being many times higher than for PECVD SiN_x.