Kinetics of Bulk Lifetime Degradation in Float‐Zone (FZ) Silico n : Fast Activation and Annihilation of Grown‐In Defects and the Role of Hydrogen vs Light

Abstract

Float-zone (FZ) silicon often has grown-in defects that are thermally activated in a broad temperature window of ∽300–800°C. These defects cause efficient electron-hole pair recombination, which deteriorates the bulk minority carrier lifetime and thereby possible photovoltaic conversion efficiencies. Little is known so far about these defects which are possibly Si-vacancy/nitrogen-related (VxNy). Here it is shown that the defect activation takes place on sub-second timescales, as does the destruction of the defects at higher temperatures. Complete defect annihilation, however, is not achieved until nitrogen impurities are effused from the wafer, as confirmed by secondary ion mass spectrometry. Hydrogenation experiments reveal the temporary and only partial passivation of recombination centers. In combination with deep-level transient spectroscopy, at least two possible defect states are revealed, only one of which interacts with H. With the help of density functional theory V1N1-centers, which induce Si dangling bonds (DBs), are proposed as one possible defect candidate. Such DBs can be passivated by H. The associated formation energy, as well as their sensitivity to light-induced free carriers, is consistent with the experimental results. These results are anticipated to contribute to a deeper understanding of bulk-Si defects, which are pivotal for the mitigation of solar cell degradation processes.