Improving the Performance of Bifacial Tunnel Oxide Passivated Contact Solar Cells: Insights into Firing-Induced Degradation Mechanisms

Tunnel oxide passivated contact (TOPCon) solar cells achieve efficiencies exceeding 26% by incorporating a heavily doped poly-Si layer with a tunnel oxide, with recent efforts focusing on enhancing the rear passivation structure. In industrial TOPCon cells, the high-temperature firing process during metal contact formation degrades the passivation quality of poly-Si/SiO_x contacts, necessitating improvements to maintain cell performance. While previous studies examine degradation factors related to the rear structure, research on mechanisms driven by the firing process remains limited. This study identifies how excess hydrogen, rather than phosphorus in-diffusion, degrades passivation quality by diffusing from SiN_x into SiO_x during the firing process. Thermal stress during the firing process dissociates c-Si/SiO_x bonds, while interstitial hydrogen accumulates at the SiO_x interface and forms hydrogen pores as defects, reducing passivation quality. To mitigate this, we introduce an Al₂O₃ layer as a hydrogen diffusion barrier, effectively preventing hydrogen diffusion into SiO_x. This approach increases the implied open-circuit voltage (iV_oc) after firing, achieving a record 729.8 mV with Al₂O₃/SiN_x double passivation layers. These findings advance the understanding of degradation mechanisms in industrial TOPCon solar cells during firing and offer practical strategies for optimizing industrial-scale solar cell manufacturing.