Light and Elevated Temperature Induced Degradation (LeTID) is one of the significant forms of degradation that have a long-term impact on the energy efficiency of silicon solar cells and modules. LeTID occurs in all types of silicon wafers used in solar cells, regardless of their crystalline structure, dopant type, or growth technique, and can lead to a relative decrease in energy efficiency of up to 14%. While the exact cause and mechanism of LeTID remain not fully understood, hydrogen-related defects are considered the primary factor. In industrial silicon (Si) cells, the hydrogenated silicon nitride (SiNx:H) which serves as both a passivation and anti-reflection coating, is the primary source of hydrogen. During the firing process, hydrogen from the SiNx:H layer diffuses into the silicon bulk, where it can act as a defect or defect precursor when exposed to light and heat. NCPRE student Resmi E., under the supervision of Dr. Sreejith K. P. and Prof. Anil Kottantharayil, studied the recombination characteristics related to LeTID in commercially viable PECVD SiNx:H passivated silicon solar cell structures with varying bulk qualities. Test structures were subjected to light soaking in a xenon test chamber under an elevated temperature of 75°C for up to 125 hours. The LeTID behavior of the test structures was characterized using photoluminescence (PL) imaging and carrier lifetime measurements. Samples showed initial degradation followed by regeneration when subjected to light and heat. Variations in open-circuit PL intensity and effective lifetime reveal that the LeTID behavior differs among samples with differing bulk qualities. The regeneration in PL intensity and effective minority carrier lifetime is relatively lower in mc-Si samples than in c-Si samples due to the high density of crystallographic defects and metallic impurities.

The study also suggests that the behavior of the surface and bulk differs with bulk qualities. However, the bulk behavior determines the overall LeTID and regeneration kinetics in both c-Si and mc-Si samples, subsequently influencing the performance parameters of the solar cells. The findings would be useful for further understanding and modeling the recombination characteristics of the solar cells at elevated temperatures, close to the normal operating temperature in field conditions. It would also be beneficial for developing mitigation measures for LeTID. The work has been published in the journal Silicon in February 2025, please see https://doi.org/10.1007/s12633-025-03254-2.