Breakdown of the Static Picture of Defect Energetics in Halide Perovskites: The Case of the Br Vacancy in CsPbBr₃

Abstract

We consider the Br vacancy in CsPbBr3 as a prototype for the impact of structural dynamics on defect energetics in halide perovskites (HaPs). Using first-principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy of the VBr level is found to be intrinsically dynamic, oscillating by as much as 1 eV on the ps time scale at room temperature. These significant energy fluctuations are correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at these Pb atomic sites. We expect this unusually strong coupling of structural dynamics and defect energetics to bear important implications for both experimental and theoretical analysis of defect characteristics in HaPs. It may also hold significant ramifications for carrier transport and defect tolerance in this class of photovoltaic materials.

Fröhlich interaction dominated by a single phonon mode in CsPbBr3

“…Therefore the proposition that defects may contribute to the observed dielectric response does not contradict the excellent PL efficiency of CsPbBr 3 NCs. The link between the dipolar relaxation and lattice defects in CsPbBr 3 is supported by molecular dynamics simulations, which have shown that the positions of lattice ions at halide vacancies fluctuate on a similar time scale (5 ps) 79 as our measured relaxation ( τ = 32 ps) (by comparison, relaxation due to Cs + motion occurs on a time scale of around 100 fs 15 ). The charge density around a defect depends strongly on the positions of neighboring ions, such that these fluctuations may induce a dipole which can respond to an electromagnetic field, as required for configurational tunneling.…”

The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing

“…[50][51][52] Moreover, these fluctuations have been shown to greatly affect the defect energy landscape. 53 Clearly, the static, relaxed structure can only represent an average picture. Because the static FA +containing structure is already greatly distorted even in the absence of dynamic effects, much more so than the Cs + -containing one, it is reasonable to assume that these differences carry over to the dynamic, room temperature systems.…”

“…[70][71][72] Moreover, these fluctuations may affect the defect energy landscape. 73 Clearly, the static, relaxed structure can only represent an average picture.…”

Small Number of Defects per Nanostructure Leads to “Digital” Quenching of Photoluminescence: The Case of Metal Halide Perovskites

“…Low temperature solution processing with very fast crystallization under the condition of kinetic competition between many processes should lead to high concentrations of crystal defects. However, somehow it is not a severe problem for device efficiency [2–5] . Here it is important to realize that not every defect leads to NR recombination, and even if some do, their efficiency can vary enormously.…”