Charge trapping, hydrogen accumulation, and structural rearrangement: A complete model for ultraviolet-induced degradation in TOPCon devices

Tunnel oxide passivated contact (TOPCon) solar cells are susceptible to ultraviolet (UV)-induced degradation (UVID), which reduces their long-term performance. This study investigates the UVID mechanism in TOPCon lifetime structures with thin (4 nm) and thick (7 nm) AlO x layers. We use a cycle of UV exposure, dark storage, and dark annealing to track changes in chemical and field-effect passivation. During UV exposure, the chemical passivation degrades, shown by an increase in the interface defect density (D it ). We attribute this to high-energy UV photons breaking Si-H bonds within the SiN x capping layer, which releases mobile hydrogen that subsequently accumulates at the interface, thereby causing recombination-active defects. In contrast, the field-effect passivation is temporarily enhanced by charge trapping in the AlO x , which increases its negative fixed charge (Q f ). A subsequent “dark storage degradation” occurs as these charges de-trap, while the chemical damage remains unchanged. During dark annealing, the accumulated hydrogen at the interface diffuses into the silicon bulk. This reduction in interfacial hydrogen concentration restores surface chemical passivation, as confirmed by a decrease in D it . Although the chemical passivation shows a full recovery, as confirmed by a decrease in D it , the FTIR analysis reveals that the complete degradation and recovery cycle induces a permanent structural rearrangement of the dielectric stack. Furthermore, the results show that the thicker 7 nm AlOx layer provides better UVID resilience. Since the field-effect passivation behaves similarly for both thicknesses, we attribute this resilience to the thicker film acting as a more effective physical barrier, reducing the transport of mobile hydrogen to the interface. This work presents a comprehensive model that links the observed UVID to specific, underlying structural changes in the passivation stack, providing guidance to address this failure mode at the solar cell level. • UVID in TOPCon is a change in both chemical degradation and a temporary field-effect enhancement. • A secondary “dark storage degradation” is caused by the de-trapping of negative charges from the AlO x layer. • Thermal recovery induces a permanent structural transformation of the passivation stack, not a simple reversal of damage. • FTIR reveals irreversible SiN x rearrangement and a chemical transformation and densification of the interfacial oxide layer during the recovery cycle. • A thicker (7 nm) AlO x layer provides superior UVID resilience by acting as a more effective hydrogen barrier.