Electronic structure of CsPbBr3 with isovalent doping and divacancies: the smallest metal Pb cluster†

Abstract

Cesium lead bromide (CsPbBr₃) has attracted considerable attention as a promising candidate for photovoltaic and optoelectronic applications owing to its relatively high defect tolerance. So far, most studies have focused on single vacancy defects while the effect of higher-order divacancy has been overlooked. Here we uncover the mechanism of formation of the Br divacancy (DV_Br), a popular type of defect that usually develops with time. We predict that DV_Br is formed via merging two isolated Br single vacancies (V_Br) and is stabilized in an apical configuration. Owing to the low formation energy of V_Br, resulting in a high population of vacancy, we predict that DV_Br could be common in CsPbBr₃, especially for those samples that are highly radiated or exposed to electric field or Br-poor environment. A single V_Br tends to be stabilized as −1 charged defect (V_Br¹⁻), forming a Pb–Pb dimer, especially in p-doped CsPbBr₃, and possesses an in-gap defective level at 0.53 eV below the bottom of the conduction band. This Pb–Pb dimer would be the smallest metal Pb cluster in CsPbBr₃, and likely in other Pb-based perovskites, and is harmful for light emission because of its deep localized defective level. Fortunately, the number of V_Br¹⁻ could be dropped through V_Br¹⁻ = V_Br⁰ + e⁻ and converted into a begin V_Br⁰ being free of in-gap defect level and an unbound Pb–Pb structure. Each DV_Br is stabilized by a redox charge transfer process via 2V_Br⁰ → V_Br¹⁺ + V_Br¹⁻, and also possesses a deep defect level, similar to V_Br¹⁻. We also demonstrate the ease of forming Cl and I substituent dopants in CsPbBr₃. Our work can inspire further studies on vacancy clusters in other metal halide perovskites.