Atomistic effects of halide mixing in all-inorganic halide perovskites

Metal halide perovskites have emerged as highly promising materials for optoelectronic applications, showcasing exceptional properties that make them ideal candidates for solar cells and light-emitting diodes. State-of-the-art perovskite compositions often involve the mixing of diverse ions, with the aim of fine-tuning the optoelectronic properties and stability of the material for the specific application. To understand the atomistic effects of ion mixing, researchers from Eindhoven University of Technology (TU/e) and Pennsylvania State University (PSU) developed a ReaxFF force field for inorganic metal halide perovskites (CsPbX₃ with X = Br or I) for use in large-scale molecular dynamics simulations [1].

The study expands on a previously developed force field for CsPbI₃, which has been useful for studying bulk degradation reactions [2] and the effects of surfaces and grain boundaries [3]. Using reference data from density functional theory calculations the researchers parameterized a ReaxFF force field for CsPb(Br_xI_1-x)₃ inorganic halide perovskites with ParAMS [4]. The newly developed I/Br/Pb/Cs parameter set marks the first time in literature that a description for Br has been made. In various benchmarks, which include equations of state, mixing enthalpies, degradation reactions, and defect migration barriers, the force field is found to perform well. In molecular dynamics simulations, the force field could accurately reproduce finite-temperature effects of the material, such as the phase transitions between the various bulk phases of the inorganic perovskites.

Using the new parameter set, the researchers established halide mixing has a profound effect on the phase transition temperature and tilting dynamics of the octahedra in the material as a result of the size mismatch of the I and Br ions. Making use of the dilute limit of halide mixing (i.e. replacing a single halide in the perovskite lattice), it was established this effect is non-local, ranging up to two nanometers away from the mixing site. The non-locality of the effects of mixing explains why small amounts of halide mixing can have large effects on material properties, such as the phase transition temperature. The new ReaxFF parameter set paves the way to further explore the complex dynamics of inorganic mixed halide perovskites with atomic simulations on large, realistic systems.

[1] Pols, M.; van Duin, A. C. T.; Calero, S.; Tao, S. Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations. J. Phys. Chem. C 2024, 128 (9), 4111–4118.

[2] Pols, M.; Vicent-Luna, J. M.; Filot, I.; van Duin, A. C. T.; Tao, S. Atomistic Insights Into the Degradation of Inorganic Halide Perovskite CsPbI3: A Reactive Force Field Molecular Dynamics Study. J. Phys. Chem. Lett. 2021, 12 (23), 5519–5525.

[3] Pols, M.; Hilpert, T.; Filot, I. A. W.; van Duin, A. C. T.; Calero, S.; Tao, S. What Happens at Surfaces and Grain Boundaries of Halide Perovskites: Insights from Reactive Molecular Dynamics Simulations of CsPbI3. ACS Appl. Mater. Interfaces 2022, 14 (36), 40841–40850.

[4] Komissarov, L.; Rüger, R.; Hellström, M.; Verstraelen, T. ParAMS: Parameter Optimization for Atomistic and Molecular Simulations. J. Chem. Inf. Model. 2021, 61 (8), 3737–3743.