Role of post-metallization anneal sequence and forming gas anneal to mitigate light and elevated temperature induced degradation of multicrystalline silicon solar cells

Abstract

The mystery surrounding the role of hydrogen in light and elevated temperature induced degradation (LeTID) of multicrystalline silicon (mc-Si) solar cells is attracting a lot of interest in the photovoltaic community. It is still unclear whether hydrogen plays a role in the degradation or the regeneration phase, with convincing evidence reported for both. In this paper, we base our work on the hypothesis that hydrogen plays a role in both phases depending on its charge state. Hydrogen in at least one charge state can passivate the LeTID defect. Thus, it is possible to mitigate LeTID in mc-Si by controlling the charge state of hydrogen. We observe in our experiments that the sequence of different thermal anneals impacts the degree of degradation. We also explore the possibility of mitigating LeTID by annealing the samples in a forming gas atmosphere. P-type mc-Si samples that received a forming gas anneal (FGA) show an improvement of the effective carrier lifetime when subjected to 1-Sun light soaking at 80 °C. Mc-Si PERC solar cells fabricated by incorporating a FGA anneal step show about 1.5% (relative) maximum drop in efficiency during 300 h of light soaking, as compared to about 3.2% (relative) maximum drop for cells fabricated by annealing in atmospheric air (without FGA), and 5.3% (relative) maximum drop for non-annealed cells. It thus seems that, after optimization, FGA is an easy to implement solution to minimise LeTID in industrial mc-Si solar cells.