bismuth oxyiodide (BiOI) has emerged as a promising candidate for lead-free perovskites. It is air-stable for over three months, shows promising early-stage photovoltaic performance and is predicted to tolerate vacancy and antisite defects.
In this study, we report on the experimental tolerance of BiOI to surface iodine compositional changes. The results show that BiOI is tolerant to iodine recombination on the surface, with the lifetime and PL intensity remaining unaffected.
The iodine content of BiOI thin films is reduced by approximately 40% during vacuum annealing (25 Pa absolute pressure) and the oxygen fraction increases by more than 45%. These changes in iodine, oxygen and bismuth compositions at the surface are accompanied by O 1s core level peaks at 531 eV and 533 eV with increasing annealing time.
However, O 2s recombination is unaffected by these compositional changes. This is attributed to the formation of deep traps, which are three to four orders of magnitude less concentrated than the surface point defects induced through vacuum annealing.
These deeper traps were also found to have significantly longer charge-carrier lifetimes than if the surface defects were recombination centers. This suggests that the surface iodine compositional changes did not affect the formation of these deep traps, and thus, they should not have an effect on the carrier concentration of the BiOI films.
