Researchers at the U.S. Department of Energy’s National Laboratory of the Rockies have assessed light- and elevated-temperature-induced degradation (LeTID) in industrial n-type tunnel oxide passivated contact (TOPCon) silicon solar cells processed with and without laser-enhanced contact optimization (LECO) and have found that LeTID-related degradation in these devices is negligible compared with the levels historically observed in passivated emitter and rear contact (PERC) solar cells.
“The previous generation of silicon PV was dominated by p-PERC cells, based on p-type Czochralski (Cz) Si wafers,” the study’s principal investigator, Paul Stradins, told pv magazine. “These wafers, if doped with boron (B), suffered from both light-induced degradation (LID) and LeTID. Replacing B with gallium (Ga) as the base dopant in the second p-PERC generation eliminated LID, but LeTID was still present. Transitioning to the current TOPCon cell generation was thought to eliminate these bulk degradation modes altogether. However, relatively recent research from the University of Konstanz found LeTID in n-Cz wafers as well, which raises potential long-term reliability concerns for TOPCon cells.”
“This potentially novel reliability concern motivated our work,” he continued. “We established that the LeTID effect is present in n-Cz wafers and TOPCon cell precursors, irrespective of dopant type. Nevertheless, the extent of degradation is relatively low due to the asymmetrical carrier capture of a LeTID defect, making it an effective recombination center in p-type Si, but less effective in n-type Si. When accelerated LeTID degradation was applied to the TOPCon solar cells, the degradation and subsequent recovery were still present. However, the effect was practically negligible when the cells were fired with sufficiently low cooling rates.”
For the tests, the scientists used n-type wafers doped with phosphorus (P), antimony (Sb), and arsenic (As). The wafers were symmetrically passivated with hydrogenated silicon nitride (SiNₓ:H), fired at 800 C, subjected to recovery treatment at 20 C under 2-sun illumination, and subsequently exposed to LeTID conditions at 120 C to 130 C under 1-sun illumination.
The researchers found that, although LeTID degradation and regeneration kinetics were similar across all three dopants, As-doped silicon exhibited approximately twice the maximum defect density of P- and Sb-doped material. They attributed this difference primarily to variations in grown-in defects and processing conditions rather than to the dopant species itself.
Overall, the team found that n-type Cz Si exhibited substantially less LeTID degradation than previously reported for p-type Ga-doped Cz Si, indicating greater intrinsic resilience to LeTID. Injection-dependent lifetime measurements further indicated that recovery is primarily associated with the disappearance or transformation of metastable bulk defects.
The analysis also revealed that regenerated samples achieved higher bulk carrier lifetimes than recovered samples, suggesting that the regeneration process can improve bulk material quality beyond the state reached after the initial recovery treatment.
The researchers also investigated recovery pre-treatment and LeTID in unmetallized and metallized TOPCon cells sourced from industrial production lines. They found that both phenomena remain observable at the cell level and can affect device performance if appropriate mitigation measures are not implemented. Their impact, however, can be substantially reduced by optimizing the firing profile and applying suitable post-processing treatments.
A direct comparison of TOPCon cells subjected to identical firing conditions, with and without LECO, showed that the process moderately suppressed changes in open-circuit voltage associated with recovery and subsequent LeTID. The researchers also found that an industrially manufactured TOPCon cell incorporating LECO experienced only limited LeTID-induced efficiency degradation, with the change remaining within acceptable warranty tolerances.
The research is described in the paper “Bulk degradation of P-, Sb- and As-doped Cz Si wafers and TOPCon cells due to light and elevated temperature,” published in Solar Energy Materials and Solar Cells.
“We believe our findings provide some metrics and guidelines for producing LeTID-stable TOPCon cells,” Stradins concluded.
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