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

2D Phase Formation on 3D Perovskite: Insights from Molecular Stiffness

Several studies have demonstrated that low-dimensional structures (e.g., two-dimensional (2D)) associated with three-dimensional (3D) perovskite films enhance the efficiency and stability of perovskite solar cells. Here, we aim to track the formation sites of the 2D phase on top of the 3D perovskite and to establish correlations between molecular stiffness and steric hindrance of the organic cations and their influence on the formation and crystallization of 2D/3D. Using cathodoluminescence combined with a scanning electron microscopy technique, we verified that the formation of the 2D phase occurs preferentially on the grain boundaries of the 3D perovskite. This helps explain some passivation mechanisms conferred by the 2D phase on 3D perovskite films. Furthermore, by employing in situ grazing-incidence wide-angle X-ray scattering, we monitored the formation and crystallization of the 2D/3D perovskite using three cations with varying molecular stiffness. In this series of molecules, the formation and crystallization of the 2D phase are found to be dependent on both steric hindrance around the ammonium group and molecular stiffness. Finally, we employed a 2D/3D perovskite heterointerface in a solar cell. The presence of the 2D phase, particularly those formed from flexible cations, resulted in a maximum power conversion efficiency of 21.5%. This study provides insight into critical aspects related to how bulky organic cations' stiffness and steric hindrance influence the formation, crystallization, and distribution of 2D perovskite phases.

Molecular dynamics simulation for phase transition of CsPbI3 perovskite with the Buckingham potential

The CsPbI3 perovskite is a promising candidate for photovoltaic applications, for which several critical phase transitions govern both its efficiency and stability. Large-scale molecular dynamics simulations are valuable in understanding the microscopic mechanisms of these transitions, in which the accuracy of the simulation heavily depends on the empirical potential. This study parameterizes two efficient and stable empirical potentials for the CsPbI3 perovskite. In these two empirical potentials, the short-ranged repulsive interaction is described by the Lennard-Jones model or the Buckingham model, while the long-ranged Coulomb interaction is summed by the damped shifted force method. Our molecular dynamics simulations show that these two empirical potentials accurately capture the γ ↔ β ↔ α and δ → α phase transitions for the CsPbI3 perovskite. Furthermore, they are up to two orders of magnitude more efficient than previous empirical models, owing to the high efficiency of the damped shifted force truncation treatment for the Coulomb interaction.

High efficiency and stability of perovskite solar cells prepared by alkali metal interfacial modification

Perovskite solar cells (PSCs) have attracted much attention at home and abroad due to their excellent photoelectric properties. Defects in the electron transport layer (ETL) and ETL/perovskite interface greatly affect the power conversion efficiency (PCE) and stability of PSCs. In the paper, the surface of tin dioxide (SnO2) ETL was modified by an alkali metal salt (NaBr, KBr, and RbBr) solution to optimize electron transport and passivate SnO2/perovskite. The results show that the photovoltaic performance of the PSCs is significantly improved after interfacial modification, especially the KBr-modified PSC has the highest PCE, which is 7.8% higher than that of the unmodified device, and the open-circuit voltage, short-circuit current density and fill factor are all greatly improved. This improvement is attributed to the fact that interfacial modification reduces the trap density of the SnO2 films, increases the mobility of the SnO2 films film, effectively passivates defects, and significantly inhibits the recombination at the SnO2/perovskite interface. This method aims to use simple and low-cost inorganic materials for effective interface modification.

Kinetic Process with Anti-Frenkel Disorder in a CsPbI3 Perovskite

Halide perovskites are rich in ionic diffusion phenomena due to their low activation energy. The soft lead iodide lattice can, in theory, endow the system with more complex defect collaborative motions. In this work, we systematically investigated the hopping mechanics of iodide interstitials with respect to various defect behaviors, such as anti-Frenkel disorder creation and annihilation. We found that the existence of iodide vacancies and interstitials can effectively lower the creation barrier of additional anti-Frenkel disorder in the halide perovskite. The free energy barriers for generating additional Frenkel defect pairs vary from 0.25 to 0.43 eV, in the proximity of those of the original iodide defects at 300 K. This finding suggests that the spontaneous creation of a specific level of anti-Frenkel disorder facilitates long-range annihilation and defect hopping processes.

Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations

All-inorganic halide perovskites have received a great deal of attention as attractive alternatives to overcome the stability issues of hybrid halide perovskites that are commonly associated with organic cations. To find a compromise between the optoelectronic properties of CsPbI3 and CsPbBr3, perovskites with CsPb(BrxI1-x)3 mixed compositions are commonly used. An additional benefit is that without sacrificing the optoelectronic properties for applications such as solar cells or light-emitting diodes, small amounts of Br in CsPbI3 can prevent the inorganic perovskite from degrading to a photo-inactive non-perovskite yellow phase. Despite indications that strain in the perovskite lattice plays a role in the stabilization of the material, a full understanding of such strain is lacking. Here, we develop a reactive force field (ReaxFF) for perovskites starting from our previous work for CsPbI3, and we extend this force field to CsPbBr3 and mixed CsPb(BrxI1-x)3 compounds. This force field is used in large-scale molecular dynamics simulations to study perovskite phase transitions and the internal ion dynamics associated with the phase transitions. We find that an increase of the Br content lowers the temperature at which the perovskite reaches a cubic structure. Specifically, by substituting Br for I, the smaller ionic radius of Br induces a strain in the lattice that changes the internal dynamics of the octahedra. Importantly, this effect propagates through the perovskite lattice ranging up to distances of 2 nm, explaining why small concentrations of Br in CsPb(BrxI1-x)3 (x ≤ 1/4) have a significant impact on the phase stability of mixed halide perovskites.

Highly Photostable Zn-Treated Halide Perovskite Nanocrystals for Efficient Single Photon Generation

Achieving pure single-photon emission is essential for a range of quantum technologies, from quantum computing to quantum key distribution to quantum metrology. Among solid-state quantum emitters, colloidal lead halide perovskite (LHP) nanocrystals (NCs) have attracted considerable interest due to their structural and optical properties, which make them attractive candidates for single-photon sources (SPSs). However, their practical utilization has been hampered by environment-induced instabilities. In this study, we fabricate and characterize in a systematic manner Zn-treated CsPbBr3 colloidal NCs obtained through Zn2+ ion doping at the Pb-site, demonstrating improved stability under dilution and illumination. The doped NCs exhibit high single-photon purity, reduced blinking on a submillisecond time scale, and stability of the bright state even at excitation powers well above saturation. Our findings highlight the potential of this synthesis approach to optimize the performance of LHP-based SPSs, opening up interesting prospects for their integration into nanophotonic systems for quantum technology applications.

Discrete Elemental Distributions inside a Single Mixed-Halide Perovskite Nanocrystal for the Self-Assembly of Multiple Quantum-Light Sources

An in-depth understanding of the structure-property relationships in semiconductor mixed-halide perovskites is critical for their potential applications in various light-absorbing and light-emitting optoelectronic devices. Here we show that during the crystal growth of mixed-halide CsPbBr1.2I1.8 nanocrystals (NCs), abundant Ruddlesden-Popper (RP) plane stacking faults are formed to release the lattice strain. These RP planes hinder the exchange of halide species across them, resulting in the presence of multiple nanodomains with discrete mixed-halide compositions inside a single CsPbBr1.2I1.8 NC. Photoluminescence peaks from these pre-segregated nanodomains, whose correlated intensity and wavelength variations signify the interactions of coupled quantum dots within a single CsPbBr1.2I1.8 NC, can be simultaneously resolved at cryogenic temperature. Our findings thus point to a fascinating scenario in which a semiconductor nanostructure can be further divided into multiple quantum-light sources, the interaction and manipulation of which will promote novel photophysics to facilitate their potential applications in quantum information technologies.

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

The migration of defects plays an important role in the stability of halide perovskites. It is challenging to study defect migration with experiments or conventional computer simulations. The former lacks an atomic-scale resolution and the latter suffers from short simulation times or a lack of accuracy. Here, we demonstrate that machine-learned force fields, trained with an on-the-fly active learning scheme against accurate density functional theory calculations, allow us to probe the differences in the dynamical behaviour of halide interstitials and halide vacancies in two closely related compositions CsPbI3 and CsPbBr3. We find that interstitials migrate faster than vacancies, due to the shorter migration paths of interstitials. Both types of defects migrate faster in CsPbI3 than in CsPbBr3. We attribute this to the less compact packing of the ions in CsPbI3, which results in a larger motion of the ions and thus more frequent defect migration jumps.

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

Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI3, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs forms in a concentration comparable to those of the most prominent point defects, whereas the other compound defects are negligible. However, under nonequilibrium thermal and operating conditions, other complexes also become as important as the point defects. Those are the Cs substituting Pb antisite, and, to a lesser extent, the compound vacancies of PbI2 or CsPbI3 units, and the I substituting Cs antisite. These compound defects only lead to shallow or inactive charge carrier traps, which testifies to the electronic stability of the halide perovskites. Under operating conditions with a quasi-Fermi level very close to the valence band, deeper traps can develop.

Grain boundary sliding and distortion on a nanosecond timescale induce trap states in CsPbBr3: ab initio investigation with machine learning force field

Grain boundaries (GBs) in perovskite solar cells and optoelectronic devices are widely regarded as detrimental defects that accelerate charge and energy losses through nonradiative carrier trapping and recombination, but the mechanism is still under debate owing to the diversity of GB configurations and behaviors. We combine ab initio electronic structure and machine learning force field to investigate evolution of the geometric and electronic structure of a CsPbBr₃ GB on a nanosecond timescale, which is comparable with the carrier recombination time. We demonstrate that the GB slides spontaneously within a few picoseconds increasing the band gap. Subsequent structural oscillations dynamically produce midgap trap states through Pb-Pb interactions across the GB. After several hundred picoseconds, structural distortions start to occur, increasing the occurrence of deep midgap states. We identify a distinct correlation of the average Pb-Pb distance and fluctuations in the ion coordination numbers with the appearance of the midgap states. Suppressing GB distortions through annealing and breaking up Pb-Pb dimers by passivation can efficiently alleviate the detrimental effects of GBs in perovskites. The study provides new insights into passivation of the detrimental GB defects, and demonstrates that structural and charge carrier dynamics in perovskites are intimately coupled.

Probing Friction Properties of Hydrogen-Free DLC Films in a Nitrogen Environment Based on ReaxFF Molecular Dynamics

In this paper, ReaxFF molecular dynamics simulations were used to look at how load and the number of nitrogen molecules affect how friction behavior in hydrogen-free diamond-like carbon (DLC). The presence of nitrogen molecules will inhibit the formation of C-C covalent bonds between the contact surfaces of the upper and lower DLC, thereby effectively suppressing the increase in friction during the initial friction phase. After the initial friction stage, the mechanical mixing of the contact surfaces brought on by the diffusion of nitrogen molecules results in considerable shear stress, which has significant impacts on the friction force. In addition, due to the existence of nitrogen molecules, the effect of graphitization of hydrogen-free DLC on friction is almost negligible.