Researchers at Yangzhou University in China have investigated ultraviolet-induced degradation (UVID) pathways in both passivated emitter and rear cell (PERC) and tunnel oxide passivated contact (TOPCon) solar cells and have concluded that this phenomenon is primarily governed by the design of the front-side passivation stack.

UVID is particularly important for TOPCon technology because its high-efficiency passivation structures rely on ultra-thin dielectric and interfacial layers that are more sensitive to UV-driven defect creation and charge accumulation, which can directly impact long-term performance and field reliability.

“UV irradiation induces silicon–hydrogen (Si–H) bond breaking and interface defect generation, whereas the optimized silicon nitride (SiNx)/aluminum oxide (AlOx) passivation structure in TOPCon solar cells maintains stable field-effect passivation and effectively suppresses recombination losses, ” corresponding author Qinqin Wang told pv magazine. “Our work further demonstrated that rational passivation-layer engineering, including optimized AlOx thickness and SiNx optical matching design, is critical for improving the long-term UV stability of TOPCon solar cells.”

The research team explained that the role of interface chemistry and hydrogen dynamics in TOPCon devices is still unexplored. In particular, the response of silicon-combined AlOx and SiNx passivation layers to UV irradiation, and their impact on hydrogen passivation and interface defect formation, is not yet fully understood

The analysis focused on interface defect density (Dit) and fixed negative charge density (Qf). Dit refers to the density of electrically active defect states at the silicon–passivation layer interface, while Qf describes the concentration of immobile negative charges embedded within the passivation layers.

The scientists used a Sinton WCT-120 metrology system for sample testing and characterization. Defect and material analyses were performed using photoluminescence (PL), electroluminescence (EL), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), ultraviolet photoelectron spectroscopy (UPS), UV–visible spectroscopy, and capacitance–voltage measurements for interface defect density (Dit) and fixed charge density (Qf).

Microstructure was examined using scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS). UV degradation testing followed IEC 61215 standards, with accelerated exposure of 60 and 120 kWh/m² at 60 C.

The group analyzed AlOx films with thicknesses of 3 nm, 5 nm, and 6 nm under UV exposure and found that thicker AlOx layers provide better stability and reduced performance degradation due to stronger field-effect passivation. It noted that, as AlOx thickness increases, the Qf becomes more negative while Dit shows a thickness-dependent response, indicating a balance between chemical and field-effect passivation.

In addition, the academics found that UV irradiation induces Si–H bond breaking and hydrogen migration, which modifies interface defects and charge states, while thicker AlOx layers stabilize oxygen-related negative charge centers more effectively.

As for SiNx, two refractive indices were studied, showing that low-index SiNx has better UV resistance due to lower UV absorption and reduced bond breaking at the interface. UV exposure transforms the SiNx network toward a more nitrogen- and oxygen-rich composition, increasing interface defect density and altering hydrogen-related bonds, the scientists said.

“Due to its passivated-contact design and the more robust field-effect passivation enabled by the SiNx/AlOx stack, TOPCon solar cells demonstrate significantly higher resistance to UV degradation than PERC cells,” Wang said. “After optimization, TOPCon devices show only a 0.74% efficiency loss following 120 kWh/m² UV exposure, whereas PERC cells experience a much larger degradation of 3.34%.”

“These findings indicate that resistance to UV degradation is primarily determined by the quality of interface passivation, underscoring the critical role of interface-engineering approaches in the development of next-generation silicon solar cells with enhanced UV stability,” he concluded.

The research findings are available in the study “Exploring the UV degradation pathways in N-TOPCon solar cells: Interface passivation and hydrogen dynamics,” published in Solar Energy Materials and Solar Cells. 

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