What Happens at Surfaces and Grain Boundaries of Halide Perovskites: Insights from Reactive Molecular Dynamics Simulations of CsPbI₃

Abstract

The commercialization of perovskite solar cells is hindered by the poor long-term stability of the metal halide perovskite (MHP) light-absorbing layer. Solution processing, the common fabrication method for MHPs, produces polycrystalline films with a wide variety of defects, such as point defects, surfaces, and grain boundaries. Although the optoelectronic effects of such defects have been widely studied, the evaluation of their impact on the long-term stability remains challenging. In particular, an understanding of the dynamics of degradation reactions at the atomistic scale is lacking. In this work, using reactive force field (ReaxFF) molecular dynamics simulations, we investigate the effects of defects, in the forms of surfaces, surface defects, and grain boundaries, on the stability of the inorganic halide perovskite CsPbI3. Our simulations establish a stability trend for a variety of surfaces, which correlates well with the occurrence of these surfaces in experiments. We find that a perovskite surface degrades by progressively changing the local geometry of PbI x octahedra from corner- to edge- to face-sharing. Importantly, we find that Pb dangling bonds and the lack of steric hindrance of I species are two crucial factors that induce degradation reactions. Finally, we show that the stability of these surfaces can be modulated by adjusting their atomistic details, by either creating additional point defects or merging them to form grain boundaries. While in general additional defects, particularly when clustered, have a negative impact on the material stability, some grain boundaries have a stabilizing effect, primarily because of the additional steric hindrance.

Compound Defects in Halide Perovskites: A First-Principles Study of CsPbI3

“…However, many crystal growth conditions are highly nonequilibrium (involving a high temperature annealing step, for instance), and defects can be formed during growth in appreciable concentrations. In molecular dynamics simulations that use a reactive force field, , applied to halide perovskites containing an appreciable amount of point defects, one often observes recombination of the latter to compound defects. From positron annihilation lifetime spectroscopy, assisted by DFT calculations, there is evidence of charge carrier trapping at compound vacancy defects in MAPbI 3 …”

How fast do defects migrate in halide perovskites: insights from on-the-fly machine-learned force fields

“…Various computational studies have provided microscopic insights into the defects, such as the type and concentration of defects in halide perovskites, 8 defect migration 9 and defectinduced degradation at perovskite grain boundaries. 10 However, the too high computational cost of ab initio methods and the limited accuracy of empirical force fields has prevented conventional computational methods from studying the subtle effects of the defect type and changes in composition on the defect migration rates, which remain an unclear and debated subject. [11][12][13][14][15] Machine-learned force fields (MLFFs) have emerged as an attractive computational method to investigate the dynamics of complex materials at an accuracy that is close to ab initio methods.…”

Blade-coating of a highly crystallized lead-free silver-bismuth halide double perovskite thin film with improved stability for high-performance photodetection

“…10,54 This behavior can be attributed to several factors, including the presence of structural disorders or dangling bonds at the grain boundary and the accumulation of point defects adjacent to the grain boundary. 57 The fabricated Cs 2 Ag-BiBr 6 thin film with reduced grain boundaries effectively reduces non-radiative recombination, which is more conducive to improving device performance.…”