Long-term stability of TOPCon solar cell precursor structures based on Ga-doped Cz-Si

In this study, the impact of tunnel oxide passivated contact (TOPCon) solar cell precursor structures on commercial Ga-doped Czochralski silicon is investigated regarding bulk- and surface-related degradation. Two sample types, symmetrical poly-Si structures and asymmetrical samples featuring various passivation stacks used in TOPCon cells are examined.

It is found that firing temperatures well above 800 °C lead to blistering and a significant reduction in performance for symmetrical TOPCon structures, reducing $i{V}_{\text{OC}}$ to below 700 mV. Treatment at an elevated temperature under constant illumination revealed that a significant degradation could only be observed at measured firing peak temperatures above 750 °C. While it is found that an AlO${}_{\text{x}}$ interlayer underneath a SiN${}_{\text{y}}$:H layer effectively reduces the extent of degradation without an (n)poly-Si layer, it seems to be less effective on top of (n)poly-Si layers.

Another experiment on the long-term stability revealed that surface-related degradation (SRD) is significantly reduced by the usage of symmetrical TOPCon structures and for a sample passivated with an SiO${}_{\text{z}}$/AlO${}_{\text{x}}$/SiN${}_{\text{y}}$:H stack. Compared to reference samples processed without TOPCon structures, a notable reduction in the extent of light- and elevated temperature-induced degradation (LeTID) is achieved in the samples featuring TOPCon structures, which is due to less hydrogen in-diffusion from passivation layer stacks into the bulk during the firing process.