Luminescence of Polybromide Defects in Cs4PbBr6

Abstract

Cs<sub>4</sub>PbBr<sub>6</sub> is a member of the halide perovskite family that is built from isolated (zero-dimensional) PbBr<sub>6</sub><sup>4-</sup> octahedra with Cs<sup>+</sup> counter ions. The material exhibits anomalous optoelectronic properties: optical absorption and weak emission in the deep ultraviolet (310 - 375 nm) with efficient luminescence in the green region (540 nm). Several hypotheses have been proposed to explain the giant Stokes shift including: (i) CsPbBr<sub>3</sub> phase impurities; (ii) self-trapped exciton; (iii) defect emission. We show -- within the modern first-principles theory of defects -- that many of the low energy point defects in Cs<sub>4</sub>PbBr<sub>6</sub> lead to the formation of polybromide (Br<sub>3</sub>) species that exist in a range of charge states. We further demonstrate from excited-state calculations that tribromide moieties are photoresponsive and can contribute to the observed green luminescence. Photoactivity of polyhalide molecules is expected to be present in other halide perovskite-related compounds.

Green-Emitting Powders of Zero-Dimensional Cs4PbBr6: Delineating the Intricacies of the Synthesis and the Origin of Photoluminescence

“…While bromine vacancy (V Br ) transition level energy in the Cs 4 PbBr 6 was calculated by Yin et al 13 to be 2.3 eV above the VBM, matching the green emission and, recently, V Br was instead suggested to form a shallow defect level. 45 In addition, as we have previously reported, 12 the intense and sharp emission and the absence of a Stokes shift does not match with deep-trap emission. In brackets, the δ iso (ppm) of the different phases is reported.…”

The Structures of Ordered Defects in Thiocyanate Analogues of Prussian Blue

“…As the Zn is four-coordinated, its bonding requirements are saturated within the layer, and so the layer is connected to the layers above and below through the Bi atom. The second layer has composition Zn 2 Bi(NCS) 6 • NCS and a pseudo-CdI 2 structure, where the Bi ('Cd') atoms approximately form a triangular lattice, in which the interstices are occupied by Zn ('I') atoms [Fig. 6(f)].…”

“…Defects are ubiquitous in functional materials and play a critical role in materials as simple as the binary rocksalt oxides 1,2 and as complex as high-temperature superconductors 3 . The importance of vacancy chemistry to molecular framework perovskites is becoming increasingly clear 4 in many important families, including the hybrid metal-halide semiconductors 5,6 , cyanide Prussian Blue analogue battery cathodes 7,8 and magnetic formate perovskites 9,10 , as the number of studies making use of defect-engineering in these materials grows. Perhaps the most widespread strategy for introducing defects is aliovalent doping, where an ion is replaced by an ion with a different charge.…”

Accumulation of Deep Traps at Grain Boundaries in Halide Perovskites

“…47 I + i forms a I I I trimer by bridging two opposite lattice iodine as shown in Figure 2c, a configuration that is also found in other perovskite-derived materials. 48 Note that the neutral iodine interstitial is predicted to be thermodynamically unstable at all Fermi levels, and thus it is not considered in the simulations. 43,44 Based on this knowledge, we placed iodine interstitials in either the negative or positive charge state at sites in the grain boundary model to investigate segregation behavior.…”