Investigating the degradation behaviours of n+-doped Poly-Si passivation layers: An outlook on long-term stability and accelerated recovery

In this article, we study the kinetics of firing-activated degradation and recovery of industrially processed n+-doped poly-Si on thin oxide passivation layers subjected to illuminated annealing at elevated temperatures. The impact of a comprehensive range of fast-firing conditions on the subsequent degradation and recovery were assessed. The results indicate that the recovery process is dependent on the peak firing temperature, where higher temperatures led to an improvement in effective lifetime of up to 20% compared to the pre-fired state, and a very low surface dark saturation current density of 2.9 fA/cm2. By cycling through light soaking and dark anneal conditions, we show that unlike the commonly studied boron-oxygen light-induced degradation (BO-LID) and light- and elevated temperature-induced degradation (LeTID), this newly observed instability in n+-doped poly-Si passivation layers is not reversible, that is, once a degradation/recovery cycle is completed, the lifetime remains very stable under subsequent light soaking. This indicates that the surface related instability may be able to be completely resolved following the completion of the first degradation/recovery cycle. With this in mind, we investigate the potential for high intensity laser illumination (up to 150 kW/m2) to rapidly increase the recovery rates, however, this does not seem sufficient to cycle through degradation and recovery on a timescale that is amenable with mass production. The mitigation of any potential instabilities at the poly-Si interface has significant implications for the reliability of n-type tunneling oxide passivated contact (TOPCon) solar cells.