Atomistic Insights Into the Degradation of Inorganic Halide Perovskite CsPbI₃: A Reactive Force Field Molecular Dynamics Study

Abstract

Halide perovskites make efficient solar cells but suffer from several stability issues. The characterization of these degradation processes is challenging because of the limited spatiotemporal resolution in experiments and the absence of efficient computational methods to study these reactive processes. Here, we present the first reactive force field for molecular dynamics simulations of the phase instability and the defect-induced degradation in CsPbI 3 . We find that the phase transitions are driven by the anharmonic fluctuations of the atoms in the perovskite lattice. At low temperatures, the Cs cations tend to move away from their preferential positions, resulting in worse contacts with the surrounding metal halide framework which initiates the conversion to a nonperovskite phase. Moreover, our simulations of defective structures reveal that, although both iodine vacancies and interstitials are mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, leading to the decomposition of perovskites to PbI 2 .

First-principles calculations of defects in metal halide perovskites: A performance comparison of density functionals

“…Whereas the addition of semi-empirical atom-pairwise vdW corrections within the D3 approach [40] have a sizable effect on the calculated DFEs of MAPbI 3 [47], an improved version based on the same scheme with Becke and Johnson damping added [DFT-D3(BJ)] [41], which provides better corrections for nonbonding distances and intramolecular interactions [55], has not been tried yet. The same holds for accurate nonlocal vdW density functionals, such as rev-vdW-DF2.…”

Common Defects Accelerate Charge Carrier Recombination in CsSnI3 without Creating Mid-Gap States

“…The iodine vacancy causes the least acceleration. This is a favorable factor, since halide vacancies are very common in MHPs. − The iodine interstitial, which often accompanies creation of the iodine vacancy, cases an intermediate acceleration. The Cs vacancy has a minor effect, since Cs atoms do not contribute to the electron and hole wave functions.…”

What Happens at Surfaces and Grain Boundaries of Halide Perovskites: Insights from Reactive Molecular Dynamics Simulations of CsPbI$_{3}$

“…We highlight that the critical steps of the degradation mechanism of perovskite surfaces (Figure 4g-i) resemble the degradation near iodine vacancies in the bulk of CsPbI 3 . 37 How-ever, we note that two distinct features make perovskite surfaces more prone to degradation:…”

“…Moreover, in some cases the V I defect causes the perovskite surface to degrade; however, this process is not restricted to perovskite surfaces and also occurs in the bulk of inorganic perovskites. 37 Whenever a V I and a V Cs defect are created in a closely spaced pair, as shown in Figure 5c-d…”

“…To do so, we perform reactive molecular dynamics simulations, using a reactive force field (ReaxFF) we developed for CsPbI3. 37 This ReaxFF force field makes use of a dynamical bond order to describe the breaking and creation of bonds. [38][39][40] The simulations allow us to characterize the evolution of the atomic species under thermal stress and to establish a stability trend of a variety of surfaces.…”