Electronic Structure of CsPbBr3 with Isovalent Doping and Divacancy: 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 relative defect tolerance characteristics. So far most of studies have focused on single vacancy defect while effect of higher-order divacancy is overlooked. Here we uncover the mechanism of formation of divacancy of Br (DVBr), a popular type of defect usually developed with time. We predict that DVBr is formed via merging two isolated Br single vacancy (VBr) and stabilized in an apical configuration. Owing to a low formation energy of VBr thus a high population of vacancy, we predict that DVBr could be common in CsPbBr3, especially for those samples being highly radiated or exposed to electric field or Br-poor environment. We find that a single VBr tends to be -1 charged (V1- Br), especially for those p-doped CsPbBr3, behaving as a polaron state of VBr which has a characteristic Pb-Pb dimmer structure. This Pb-Pb dimer, absent in V0 Br, is the smallest metal Pb cluster in CsPbBr3 and harmful for light emission because of a localized defective level associated with. In contrast, DVBr generates no trap level and is benign, probably due to a negative-U behavior of its source VBr as indicative of a two-electron filling (+/1) transition level in the band gap of the latter. Fortunately, the number of undesired V1- Br could be dropped through emerging each two VBr into a DVBr, or reducing V1- Br via V1- Br = V0 Br + e- to make it converted into V0 Br which has an unbound Pb-Pb structure. Both DVBr and V0 Br are free of trap level, and the higher-order trap-free DVBr maybe one of the ultimate reasons accounting for the defective tolerant nature of CsPbBr3. We also prove the ease of forming Cl and I substituting dopants in CsPbBr3. Our work can stimulate studies on vacancy clusters in other metal halide perovskites.