Assessing the Impact of Static Instability on Intrinsic Defects in Photovoltaic Material CuBiSCl ₂ by First-Principles Molecular Dynamics Simulation

Defects in photovoltaic materials often induce detrimental charge recombination centers or midgap trap states, severely degrading device performance. Defect tolerance is a desired property that enables photovoltaic (PV) materials to mitigate energy losses from Shockley–Read–Hall recombination. CuBiSCl2 is a promising PV material possessing the right band gap and probably exhibiting defect tolerance. Previous studies suggested that there could exist deep-level intrinsic defects, such as sulfur vacancies (vS), in CuBiSCl2. Here, we demonstrate that CuBiSCl2 exhibits static instability, i.e., exhibiting significant imaginary frequencies in the phonon spectrum calculated at 0 K. This could affect the calculated defect properties employing the conventional method. To study whether the deep defect levels obtained at 0 K result from the static instability, we first selected an approach for accurately obtaining the atomic structure of CuBiSCl2 and then employed molecular dynamics (MD) to carry out ensemble averages to study defect properties. Our results suggest that vS is indeed a deep-level defect, and the static instability issue is insignificant on the defect transition levels in this material. We also attempted to use MD-averaged structures for static defect calculations so that high-level methods, such as hybrid functionals with spin–orbit coupling, can be used. However, this approach was found to not be applicable in predicting the transition levels of certain defects, e.g., copper interstitial defects, due to the highly displacing Cu atoms at room temperature.