Defects in polymorphous cubic halide perovskites: coexistence of shallow and deep vacancy transition levels

Halide perovskites are usually said to be defect tolerant materials suggesting a possible explanation for being good solar compounds. Molecular dynamics (MD) simulations of Cohen et al revealed that the orbital energy of the Br-vacancy in CsPbBr₃ resides deep in the gap (i.e., not defect tolerant) and can considerably fluctuate on the time scale of the MD trajectories. Recently, it was pointed out that the static structure of cubic halide perovskites involves a distribution of different local environments such as tilting and atomic displacements, being a polymorphous network [Zhao, X., Dalpian, G. M., Wang, Z. & Zunger, A. The polymorphous nature of cubic halide perovskites. arXiv:1905.09141 (2019)]. We have studied anion vacancies in different, inequivalent lattice sites in the polymorphous network of CsSnI₃, CsPbI₃, MaPbI_3, and CsPbBr₃, finding that all except CsPbBr₃ have shallow transition levels whereas the latter has a bimodal distribution with some deep levels (mostly shallow). We show how these results correlate with the local geometric deformations of the various sites in the polymorphous network.