Degradation and Recovery of n-Type Multi-Crystalline Silicon Under Illuminated and Dark Annealing Conditions at Moderate Temperatures

Recently, an n-type multi-crystalline silicon (mc-Si) was observed to be susceptible to degradation under illumination at elevated temperatures with similarities to carrier-induced degradation in p-type mc-Si. In this study, we demonstrate degradation and regeneration of the effective lifetime of non-diffused n-type mc-Si wafers using illuminated and dark annealing conditions at moderate temperatures. Under illuminated annealing conditions, the degradation and regeneration rates of the n-type mc-Si are observed to be slower than those of the p-type mc-Si; however, the opposite trend was observed under dark annealing conditions. The carrier-induced degradation kinetics of the n-type wafers can be described by degradation and regeneration that occur simultaneously, and the activation energies have been identified to be 1.23 ± 0.16 eV for the degradation process and 1.34 ± 0.08 eV for the regeneration. Surprisingly, no degradation was observed in n-type mc-Si under dark annealing above 160 °C. Rather, at these conditions, a two-stage improvement in the lifetime was observed. Although degradation occurs after a subsequent laser treatment, the stable lifetime at the end of the degradation is still slightly higher than its initial value.

I. Introduction

Multi-Crystalline silicon (mc-Si) wafers are known to degrade under illumination at elevated temperature [1]–[8] and under dark annealing [9]–[12]. This degradation is often named light and elevated temperature induced degradation (LeTID) [2], [3], [5], [13], [14]; however, as it has been reported that light is not required for its formation, is also called carrier-induced degradation (CID) [2], [5], as it seems that excess carriers are the main requirement for this degradation. Other forms of light-induced degradation, such as the formation of the boron–oxygen (B–O) complex or iron–boron (Fe–B) pair dissociation, cannot explain this degradation of carrier lifetime, as CID has been observed also in gallium-doped mc-Si wafers, and its time scales are significantly slower compared with B–O degradation [1]. Under standard field operation conditions, the degradation and recovery cycle of CID in p-type mc-Si solar cells has an extremely long timescale from 5 to 20 years, depending on the location of the photovoltaic (PV) system [2], and would result in a significant energy yield loss over this period (because of a drop in efficiency of up to 16% relative) [15]. The shift of the silicon (Si) PV industry toward passivated emitter rear cells (PERC) fabricated using p-type mc-Si substrates [16] is being challenged by this degradation. To date, the root cause of CID has not been clarified and, therefore, remains a key area of research focus.