Comparative study of degradation/regeneration related to boron-oxygen defects in p-type silicon-based solar cells with standard and perc structures

Abstract

Despite significant advancements in the efficiency of solar cells across diverse architectural designs, the photovoltaic (PV) community continues to face the challenge of sustaining peak performance throughout the operational lifetime of solar panels under real-world conditions. Defects that arise during crystal growth or become activated during solar cell operation contribute to issues such as lifetime degradation and performance instability. A notable degradation mechanism, light- and elevated temperature-induced degradation (LeTID), substantially diminishes the performance of solar cells in field conditions. Consequently, it is imperative to identify and develop industrial solutions that can effectively mitigate such degradation. This thesis aims to expand the current understanding of degradation mechanisms in commercial silicon solar cells, with a particular focus on developing strategies to mitigate LeTID prior to deployment in the field. The results of this study show that the extent of LeTID degradation is strongly influenced by the solar cell architecture. In particular, the relative efficiency loss caused by LeTID is estimated at about 6% for commercial Passivated Emitter and Rear Contact (PERC) cells made of p-type boron-doped silicon, while it is about 2.5% for multicrystalline silicon (mc-Si) cells, according to our experimental results. To address this issue, the thesis proposes a two-step mitigation approach involving two distinct treatment processes: one group of cells underwent pre-annealing, while another group was subjected to electrical injection annealing. Both methods were explored as potential solutions for mitigating LeTID in completed p-type Czochralski silicon (Cz-Si) PERC solar cells and mc-Si Al-BSF solar cells. The electrical injection annealing process, in particular, was shown to significantly reduce degradation, leading to a relative reduction in efficiency losses (?) by 1-3%.