Lab tests find negligible LeTID in TOPCon solar cells

At 120 C to 130 C, under one-sun illumination, researchers from the US Department of Energy’s National Laboratory of the Rockies accelerated a form of solar-cell ageing known as LeTID — light- and elevated-temperature-induced degradation. Industrial n-type TOPCon cells, tested with and without laser-enhanced contact optimization (LECO), showed only limited efficiency loss. The researchers describe the degradation as practically negligible when the cells were fired with sufficiently low cooling rates.

That matters because TOPCon — tunnel oxide passivated contact — is now being examined against a problem that marked the previous PERC generation, or passivated emitter and rear contact cells. Boron-doped p-type silicon suffered both light-induced degradation and LeTID. Replacing boron with gallium removed light-induced degradation, but LeTID remained. More recent work found the effect in n-type silicon too, raising questions about TOPCon’s long-term reliability.

The new experiments used n-type wafers doped with phosphorus, antimony and arsenic. They were fired at 800 C, exposed to recovery treatment at 20 C under two-sun illumination, then placed under LeTID conditions. The defect behaved similarly across the three dopants, but arsenic-doped silicon reached approximately twice the maximum defect density of the phosphorus- and antimony-doped material. The researchers linked that difference mainly to defects formed during growth and processing, rather than to the dopant itself.

The effect can still be seen in both unfinished and metallized TOPCon cells from industrial production lines. But the tests also point to practical levers: manufacturers can optimize the firing profile and add post-processing treatments. LECO moderately suppressed changes in open-circuit voltage during recovery and later LeTID, while an industrial LECO cell showed efficiency degradation within acceptable warranty tolerances, according to the study.