Electronic Structure and Optoelectronic Properties of Bismuth Oxyiodide Robust against Percent-Level Iodine-, Oxygen-, and Bismuth-Related Surface Defects

In the search for nontoxic alternatives to lead-halide perovskites, bismuth oxyiodide (BiOI) has emerged as a promising contender. BiOI is air-stable for over three months, demonstrates promising early-stage photovoltaic performance and, importantly, is predicted from calculations to tolerate vacancy and antisite defects. Here, whether BiOI tolerates point defects is experimentally investigated. BiOI thin films are annealed at a low temperature of 100 degrees C under vacuum (25 Pa absolute pressure). There is a relative reduction in the surface atomic fraction of iodine by over 40%, reduction in the surface bismuth fraction by over 5%, and an increase in the surface oxygen fraction by over 45%. Unexpectedly, the Bi 4f(7/2) core level position, Fermi level position, and valence band density of states of BiOI are not significantly changed. Further, the charge-carrier lifetime, photoluminescence intensity, and the performance of the vacuum-annealed BiOI films in solar cells remain unchanged. The results show BiOI to be electronically and optoelectronically robust to percent-level changes in surface composition. However, from photoinduced current transient spectroscopy measurements, it is found that the as-grown BiOI films have deep traps located approximate to 0.3 and 0.6 eV from the band edge. These traps limit the charge-carrier lifetimes of BiOI, and future improvements in the performance of BiOI photovoltaics will need to focus on identifying their origin. Nevertheless, these deep traps are three to four orders of magnitude less concentrated than the surface point defects induced through vacuum annealing. The charge-carrier lifetimes of the BiOI films are also orders of magnitude longer than if these surface defects were recombination active. This work therefore shows BiOI to be robust against processing conditions that lead to percent-level iodine-, bismuth-, and oxygen-related surface defects. This will simplify and reduce the cost of fabricating BiOI-based electronic devices, and stands in contrast to the defect-sensitivity of traditional covalent semiconductors.