UV-induced degradation in TOPCon solar cells: Hydrogen dynamics and impact of UV wavelength

Tunnel oxide passivated contact (TOPCon) solar cells dominate the worldwide photovoltaic market due to their high efficiency and low manufacturing costs. However, recent reliability studies have shown that TOPCon solar cells and modules can degrade significantly under ultraviolet (UV) radiation, known as UV-induced degradation (UVID). The effect of UV radiation on TOPCon solar cells, particularly concerning the role of hydrogen and wavelength dependence in the UV range, remains ambiguous, highlighting the need for further investigation. This study uses UV-A and UV-B exposure to understand the impact of UV on TOPCon precursors and lifetime structures. We demonstrate that UV-B and UV-A exposures lead to the same extent of degradation, although the process occurs significantly faster under UV-B, indicating that UV-B can be used for accelerated UV degradation testing. Notably, no Light- and elevated Temperature-Induced Degradation (LeTID) was observed when samples were exposed to UV radiation; in contrast, LeTID was observed when samples were exposed to the same temperatures without UV radiation. This suggests that UV radiation not only alters the surface but also influences hydrogen dynamics within the bulk, consistent with LeTID being a hydrogen-related bulk defect. Time of flights secondary ion mass spectrometry analysis further supports these findings by revealing an increased hydrogen concentration at the AlO_x/(p⁺)Si interface after UV exposure. This can be attributed to the fact that UV radiation is able to break Si-H bonds, where the free hydrogen can redistribute into a bonded or unbonded state in the sample. In addition, the breaking of Si-H bonds at the silicon interface increases the interface defect density and, thus, surface recombination. The rear side of the TOPCon precursor exhibited no significant degradation, as the poly-Si layer effectively absorbs UV radiation below 370 nm, shielding the tunnel oxide layer and rendering the rear side relatively resistant to UVID. This work contributes to the understanding of UVID mechanisms in TOPCon solar cells and provides insights for developing effective mitigation strategies.