Impact of firing and capping layers on long-term stability of doped poly-Si passivating contact layers

Polysilicon based passivating contacts for solar cells are expected to gain significant market share in the future. To ensure successful deployment, understanding the long-term stability of any new product is essential. Degradation and recovery of lifetime can occur in the bulk and surface-related components of a solar cell. In this work, we use planar p-type Czochralski silicon (20 Ω.cm) to fabricate symmetrical lifetime samples featuring p- or n-poly-Si layers capped by AlOx or SiNy to study the impact of the capping layer under different firing conditions, followed by annealing and illumination, on degradation mechanisms. Initially, AlOx samples perform better than SiNy samples. After firing at different temperatures below 792 °C, the passivation quality of SiNy samples improves significantly, beyond that of AlOx capped samples. During accelerated degradation (dark annealing and light soaking under high intensity illumination), AlOx samples degrade slightly while SiNy samples demonstrate a significant degradation (directly correlated to the firing temperature), which is followed by recovery. This behaviour is similar to light- and elevated temperature-induced degradation in passivated emitter and rear solar cells, suggesting a possible hydrogen-related mechanism. Lifetime analysis shows that the strongest changes occur in low injection levels, while the dark saturation current remains virtually the same.