Impact of Solar Cell Processing on LeTID in Crystalline Silicon

Abstract

In this work, the process influences during solar cell production on the light and elevated temperature-induced degradation (LeTID) are explored. A special focus lies on the impact of state-of-the-art crystal growth by the Czochralski method with melt recharging (RCz-Si) and on the stability of p-type interdigitated back-contact (pIBC) solar cells.

Chapter 3 discusses the fundamentals of segregation in the RCz-Si process. The maximum defect density is found to increase progressively in later ingot pulls, likely due to impurity incorporation or intrinsic defect generation. However, no correlation is found between initially present hydrogen dimers and degradation extent, ruling out hydrogen as the primary driver. Instead, an unknown species accumulating in the melt is suspected to cause the increased degradation.

Chapter 4 focuses on the defect kinetics in n-type Cz-Si under light soaking and LeTID conditions, which seem to follow two independent three-state models. Both phosphorus- and antimony-doped RCz-Si exhibit regeneration-dominated long-term stability behavior at illumination at elevated temperatures.

Chapter 5 demonstrates a correlation between peak firing temperature and maximum cooling rate. The maximum defect density correlates with initial hydrogen dimer concentration, independent of the capping layer composition, indicating (n)poly-Si alone serves as an effective hydrogen sink. Lastly, a pIBC solar cell on Ga-doped Cz-Si with TOPCon passivation and slow belt speed during firing showed strong LeTID suppression. Transient light-soaking effects were observed independently of bulk degradation and may be linked to localized improvements in surface passivation.

Although intrinsic material quality and hydrogen-related processing factors often act independently, their combined effect ultimately determines the overall extent of degradation in silicon solar cells.