Excessive light-induced degradation in boron-doped Cz silicon PERC triggered by dark annealing

This work investigates the impact of annealing at elevated temperatures on the light-induced degradation (LID) of passivated emitter and rear cells (PERC) processed on boron-doped Czochralski-grown silicon substrates. The boron-oxygen (BO) defect has been stabilised prior to annealing and subsequent LID treatment. Excessive LID of up to 19.1 %rel. is observed upon illumination after extended dark annealing at 150 °C for 552 h, which is well above the BO defect-related LID of 5.6 %rel. measured upon illumination after cell processing if BO is not stabilised. Light and elevated Temperature Induced Degradation (LeTID), iron-boron pairs and surface recombination are excluded as root causes for the observed increased LID, which shows a similar behaviour as the BO defect but which cannot be explained by the well-established three-state model of the BO defect with the assumption of an empty regenerated state prior to BO stabilisation. Two speculative hypothesis for an explanation are (i) that further BO defect precursors are formed, which could be described via a reservoir or (ii) that a high percentage of the in-principle available BO defects are already in the stabilised state even without dedicated BO defect stabilisation. This increased LID does not occur when at least a small level of excess carrier concentration is induced during extended annealing and, hence, is expected not to occur during field operation. However, the observed behaviour is highly relevant for accelerated aging testing such as, e.g., damp heat testing during IEC and UL certification.