The Centre of New Technologies, University of Warsaw invites to a seminar by
Prof. Claudio Quarti
Laboratory for Chemistry of Novel Materials, University of Mons, Belgium
Atomistic simulations of chemically disordered compounds: the curious case of mixed halide perovskites
Date: 22nd September 2023, Friday
Time: 12:00 pm (Central European Time)
Host: dr hab. Silvio Osella
The seminar will be in the CeNT aula hall (00.142) on the main floor.
Abstract: Halide perovskites are emerging as interesting candidates for opto-electronic applications. Characterized by ABX3 chemical composition (X=halide, B=divalent metal and A=small organic cation or an alkali atom), these materials present at the time both the excellent electronic properties of conventional semiconductors (long-living charge carriers with light mass, small exciton binding energy, etc.) and the broad compositional “tailorability” of organic semiconductors. The light absorption onset in particular can be tuned via the composition of the halide site, Noh et al. first demonstrating full color tunability from red to blue, by exploring compositions ranging from pure lead-iodide to bromine, chlorine, throughout intermediate, mixed halide compositions.[1] This finding paved the way for preparation of perovskite-based Light-Emitting Devices (LEDs) covering all emission spectrum,[2-4] as well as tandem solar cells.[5-6]
Despite the evident technological success of mixed halide perovskites, a fundamental question arises, i.e., how can we reconcile the impressive performances from Refs. [1-6] with the intrinsic compositional inhomogeneity of the halide site, which is a source of disorder. To address this point, we performed Density Functional Theory simulations on periodic models of mixed iodine/bromine models, at zero Kelvin.[7-8] These point out that halide substitution does not lead to formation of trap states, both in the dilute limit and in the case of homogeneously distributed iodine/bromine ions. Only in the case of halide segregation, the valence band maximum localizes in the iodine-rich domain, the bromine-rich domain acting as a barrier to hole diffusion.[7] We then accounted for thermally-induced ion-motion resorting to DFT-based molecular dynamics simulations.[8] This allows to grasp the influence on the electronic properties due to “static” compositional disorder from iodine/bromine mixing and “dynamic” structural disorder associated to phonons, at once. Band gap broadening resulting from real-time ion dynamics falls in reasonable agreement with linewidth from photoluminescence,[9] finite-temperature simulations showing very small inhomogeneous contribution to broadening, both for pure halide and homogeneously dispersed iodine/bromine compositions. This is no longer the case for models featuring halide segregation, which show wider band gap fluctuations and large inhomogeneous contribution to broadening. This peculiar behavior is associate to the lattice mismatch at the iodine-rich/bromine-rich interface.
All in all, the present calculations shed light on the mechanisms through which compositional disorder from halide mixing influences the electronic properties of halide perovskites. In addition, they show how atomistic simulations may provide a detailed understanding of the properties of complex systems, including the interpretation of physical observables, like light emission linewidths, under various conditions.
