Edge Influence on Charge Carrier Localization and Lifetime in CH3NH3PbBr3 Perovskite: Ab Initio Quantum Dynamics Simulation

Unraveling the excitonics of light emission from metal-halide perovskite quantum dots

Metal halide semicondictor perovskites have been under intense investigation for their promise in light absorptive applications like photovoltaics. They have more recently experienced interest for their promise in light emissive applications. A key aspect of perovskites is their glassy, ionic lattice that exhibits dynamical disorder. One possible result of this dynamical disorder is their strong coupling between electronic and lattice degrees of freedom which may confer remarkable properties for light emission such as defect tolerance. How does the system, comprised of excitons, couple to the bath, comprised of lattice modes? How does this system-bath interaction give rise to novel light emissive properties and how do these properties give insight into the nature of these materials? We review recent work from this group in which time-resolved photoluminescence spectroscopy is used to reveal such insights. Based upon a fast time resolution of 3 ps, energy resolution, and temperature dependence, a wide variety of insights are gleaned. These insights include: lattice contributions to the emission linewidths, multiexciton formation, hot carrier cooling, excitonic fine structure, single dot superradiance, and a breakdown of the Condon approximation, all due to complex structural dynamics in these materials.

Carrier transport in bulk and two-dimensional Zn2(V,Nb,Ta)N3 ternary nitrides

Density functional theory-based simulations are applied to study the electronic structures, carrier masses, carrier mobility and carrier relaxation times in bulk and two-dimensional (2D) Zn2(V,Nb,Ta)N3 ternary nitrides. Bulk Zn2(V,Nb,Ta)N3 possess moderate band gap sizes of 2.17 eV, 3.11 eV, and 3.40 eV, respectively. Two-dimensional Zn2(V,Nb,Ta)N3 have slightly higher band gap sizes of 2.77 eV, 3.33 eV, and 3.23 eV, respectively. Carrier mass, carrier mobility and carrier relaxation time are found to be anisotropic in all the studied structures. Bulk and 2D samples show an order of magnitude higher electron mobility compared to hole mobility. The highest electron mobility in bulk Zn2NbN3 and Zn2TaN3 is about ∼103 cm2 V-1 s-1. Importantly, for 2D Zn2NbN3, an abnormally high electron mobility of 1.67 × 104 cm2 V-1 s-1 is observed, which is not inferior to the highest known electron mobility values in 2D materials. Such a high electron mobility in 2D Zn2NbN3 can be attributed to a strong delocalization of the conduction band minimum, which is responsible for electron transport. Therefore, this work opens up new materials for high performance nanodevices, such as tandem solar cells and field-effect transistors. This study also provides deep physical insights into the nature of carrier transport mechanisms in bulk and 2D Zn2(V,Nb,Ta)N3 ternary nitrides.

Prolonged Exciton Lifetime Is Achieved in Porphyrin Nanoring by Template Engineering: A Nonadiabatic Tight Binding Approach

Porphyrin nanoring has been attracting immense attention due to its light harvesting capacity and potential applications in optical, catalysis, sensor, and electronic devices. We demonstrate by nonadiabatic quantum dynamics simulations that the photovoltaic efficiency can be enhanced by template engineering. Altering the hexadentate template (T6) with two tridentate templates (2T3) within the porphyrin ring (P6) cavity accelerated the electron transfer twice and suppressed the electron-hole recombination by nearly three times. The atomistic tight-binding simulation rationalized the dynamics by different localizations of charge of the band edge states, changes in nonadiabatic coupling, alteration in quantum coherence, and involvement of diverse electron-phonon vibrational modes. Further 2T3 templates more strongly hold the P6 ring than T6, reducing the structural fluctuation. As a result, the nonadiabatic coupling becomes weaker and suppresses the carrier recombination. Current atomistic simulation presents a template engineering strategy to enhance the exciton lifetime along with ultrafast charge separation, crucial factors for photovoltaic applications.

Compression Eliminates Charge Traps by Stabilizing Perovskite Grain Boundary Structures: An Ab Initio Analysis with Machine Learning Force Field

Grain boundaries (GBs) play an important role in determining the optoelectronic properties of perovskites, requiring an atomistic understanding of the underlying mechanisms. Strain engineering has recently been employed in perovskite solar cells, providing a novel perspective on the role of perovskite GBs. Here, we theoretically investigate the impact of axial strain on the geometric and electronic properties of a common CsPbBr3 GB. We develop a machine learning force field and perform ab initio calculations to analyze the behavior of GB models with different axial strains on a nanosecond time scale. Our results demonstrate that compressing the GB efficiently suppresses structural fluctuations and eliminates trap states originating from large-scale distortions. The GB becomes more amorphous under compressive strain, which makes the relationship between the electronic structure and axial strain nonmonotonic. These results can help clarify the conflicts in perovskite GB experiments.

Ab Initio Quantum Dynamics Simulation of the Impact of Graphene on the Carrier Lifetime of the ZnV2O6 Photocatalyst

We used a nonadiabatic molecular dynamics simulation to determine the carrier dynamics of a graphene/ZnV2O6 heterostructure in the search for an effective photocatalyst material. The C-2p orbital promotes the wave function overlap, guiding electrons to move between graphene and ZnV2O6, successfully achieving good mixing with the valence and conduction bands in ZnV2O6 materials, which is conducive to supporting carrier migration. The overlap between graphene/ZnV2O6 electrons and hole wave functions is less than that of ZnV2O6, and there is small absolute nonadiabatic coupling. The charge separation caused by graphene increases the carrier lifetime and prevents nonradiative electron-hole recombination. This study reveals the microscopic mechanism of extending the carrier lifetime of ZnV2O6 by introducing graphene, providing useful insights for regulating the electronic structure, promoting electron transfer and ultrafast electron and hole transfer. This strategy provides design considerations for advanced photocatalytic materials.

Breaking the Condon Approximation for Light Emission from Metal Halide Perovskite Nanocrystals

The idea that the electronic transition dipole moment does not depend upon nuclear excursions is the Condon approximation and is central to most spectroscopy, especially in the solid state. We show a strong breakdown of the Condon approximation in the time-resolved photoluminescence from CsPbBr3 metal halide perovskite semiconductor nanocrystals. Experiments reveal that the electronic transition dipole moment increases on the 30 ps time scale due to structural dynamics in the lattice. Ab initio molecular dynamics calculations quantitatively reproduce experiments by considering excitation-induced structural dynamics.

The sp3 Defect Decreases Charge Carrier Lifetime in (8,3) Single-Walled Carbon Nanotubes

A recent experimental approach introduces sp3 defects into single-walled carbon nanotubes (SWNTs) through controlled functionalization with guanine, resulting in a decrease in charge carrier lifetime. However, the physical mechanism behind this phenomenon remains unclear. We employ nonadiabatic molecular dynamics to systematically model the nonradiative recombination process of electron-hole pairs in SWNTs with sp3 defects generated by a guanine molecule. We demonstrate that the introduction of sp3 defects creates an overlapping channel between the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO), significantly enhancing the nonadiabatic (NA) coupling and leading to a 4.7-fold acceleration in charge carrier recombination compared to defect-free SWNTs. The charge carrier recombination slows significantly at a lower temperature (50 K) due to the weakening of the NA coupling. Our results rationalize the accelerated recombination of charge carriers in SWNTs with sp3 defects in experiments and contribute to a deeper understanding of the carrier dynamics in SWNTs.

Efficient Modeling of Quantum Dynamics of Charge Carriers in Materials Using Short Nonequilibrium Molecular Dynamics

Nonadiabatic molecular dynamics provides essential insights into excited-state processes, but it is computationally intense and simplifications are needed. The classical path approximation provides critical savings. Still, long heating and equilibration steps are required. We demonstrate that practical results can be obtained with short, partially equilibrated ab initio trajectories. Once the system's structure is adequate and essential fluctuations are sampled, the nonadiabatic Hamiltonian can be constructed. Local structures require only 1-2 ps trajectories, as demonstrated with point defects in metal halide perovskites. Short trajectories represent anharmonic motions common in defective structures, an essential improvement over the harmonic approximation around the optimized geometry. Glassy systems, such as grain boundaries, require different simulation protocols, e.g., involving machine learning force fields. 10-fold shorter trajectories generate 10-20% time scale errors, which are acceptable, given experimental uncertainties and other approximations. The practical NAMD protocol enables fast screening of excited-state dynamics for rapid exploration of new materials.

Rapid Interlayer Charge Separation and Extended Carrier Lifetimes due to Spontaneous Symmetry Breaking in Organic and Mixed Organic-Inorganic Dion-Jacobson Perovskites

Promising alternatives to three-dimensional perovskites, two-dimensional (2D) layered metal halide perovskites have proven their potential in optoelectronic applications due to improved photo- and chemical stability. Nevertheless, photovoltaic devices based on 2D perovskites suffer from poor efficiency owing to unfavorable charge carrier dynamics and energy losses. Focusing on the 2D Dion-Jacobson perovskite phase that is rapidly rising in popularity, we demonstrate that doping of complementary cations into the 3-(aminomethyl)piperidinium perovskite accelerates spontaneous charge separation and slows down charge recombination, both factors improving the photovoltaic performance. Employing ab initio nonadiabatic (NA) molecular dynamics combined with time-dependent density functional theory, we demonstrate that cesium doping broadens the bandgap by 0.4 eV and breaks structural symmetry. Assisted by thermal fluctuations, the symmetry breaking helps to localize electrons and holes in different layers and activates additional vibrational modes. As a result, the charge separation is accelerated. Simultaneously, the charge carrier lifetime grows due to shortened coherence time between the ground and excited states. The established relationships between perovskite composition and charge carrier dynamics provide guidelines toward future material discovery and design of perovskite solar cells.

Photolysis versus Photothermolysis of N2O on a Semiconductor Surface Revealed by Nonadiabatic Molecular Dynamics

Identifying photolysis and photothermolysis during a photochemical reaction has remained challenging because of the highly non-equilibrium and ultrafast nature of the processes. Using state-of-the-art ab initio adiabatic and nonadiabatic molecular dynamics, we investigate N₂O photodissociation on the reduced rutile TiO₂(110) surface and establish its detailed mechanism. The photodecomposition is initiated by electron injection, leading to the formation of a N₂O^- ion-radical, and activation of the N₂O bending and symmetric stretching vibrations. Photothermolysis governs the N₂O dissociation when N₂O^- is short-lived. The dissociation is activated by a combination of the anionic excited state evolution and local heating. A thermal fluctuation drives the molecular acceptor level below the TiO₂ band edge, stabilizes the N₂O^- anion radical, and causes dissociation on a 1 ps timescale. As the N₂O^- resonance lifetime increases, photolysis becomes dominant since evolution in the anionic excited state activates the bending and symmetric stretching of N₂O, inducing the dissociation. The photodecomposition occurs more easily when N₂O is bonded to TiO₂ through the O rather than N atom. We demonstrate further that a thermal dissociation of N₂O can be realized by a rational choice of metal dopants, which enhance p-d orbital hybridization, facilitate electron transfer, and break N₂O spontaneously. By investigating the charge dynamics and lifetime, we provide a fundamental atomistic understanding of the competition and synergy between the photocatalytic and photothermocatalytic dissociation of N₂O and demonstrate how N₂O reduction can be controlled by light irradiation, adsorption configuration, and dopants, enabling the design of high-performance transition-metal oxide catalysts.

Control of Charge Carrier Relaxation at the Au/WSe2 Interface by Ti and TiO2 Adhesion Layers: Ab Initio Quantum Dynamics

Phonon-mediated charge relaxation plays a vital role in controlling thermal transport across an interface for efficient functioning of two-dimensional (2D) nanostructured devices. Using a combination of nonadiabatic molecular dynamics with real-time time-dependent density functional theory, we demonstrate a strong influence of adhesion layers at the Au/WSe₂ interface on nonequilibrium charge relaxation, rationalizing recent ultrafast time-resolved experiments. Ti oxide layers (TiO_x) create a barrier to the interaction between Au and WSe₂ and extend hot carrier lifetimes, creating benefits for photovoltaic and photocatalytic applications. In contrast, a metallic Ti layer accelerates the energy flow, as needed for efficient heat dissipation in electronic devices. The interaction of metallic Ti with WSe₂ causes W-Se bond scissoring and pins the Fermi level. The Ti adhesion layer enhances the electron-phonon coupling due to an increased density of states and the light mass of the Ti atom. The conclusions are robust to presence of typical point defects. The atomic-scale ab initio analysis of carrier relaxation at the interfaces advances our knowledge in fabricating nanodevices with optimized electronic and thermal properties.

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.

Fluctuations at Metal Halide Perovskite Grain Boundaries Create Transient Trap States: Machine Learning Assisted Ab Initio Analysis

All-inorganic perovskites are promising candidates for solar energy and optoelectronic applications, despite their polycrystalline nature with a large density of grain boundaries (GBs) due to facile solution-processed fabrication. GBs exhibit complex atomistic structures undergoing slow rearrangements. By studying evolution of the Σ5(210) CsPbBr₃ GB on a nanosecond time scale, comparable to charge carrier lifetimes, we demonstrate that GB deformations appear every ∼100 ps and increase significantly the probability of deep charge traps. However, the deep traps form only transiently for a few hundred femtoseconds. In contrast, shallow traps appear continuously at the GB. Shallow traps are localized in the GB layer, while deep traps are in a sublayer, which is still distorted from the pristine structure and can be jammed in unfavorable conformations. The GB electronic properties correlate with bond angles, with notable exception of the Br-Br distance, which provides a signature of halide migration along GBs. The transient nature of trap states and localization of electrons and holes at different parts of GBs indicate that charge carrier lifetimes should be long. At the same time, charge mobility can be reduced. The complex, multiscale evolution of geometric and electronic structures of GBs rationalize the contradictory statements made in the literature regarding both benign and detrimental roles of GBs in perovskite performance and provide new atomistic insights into perovskite properties.

Anion Doping Delays Nonradiative Electron-Hole Recombination in Cs-Based All-Inorganic Perovskites: Time Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we demonstrate that composition engineering of the X-site anions has a strong influence on the nonradiative electron-hole recombination and thermodynamic stability of cesium-based all-inorganic perovskites. Partial substitution of iodine(I) with bromine (Br) and acetate (Ac) anions reduces the NA electron-vibrational coupling by minimizing the overlap between the electron and hole wave functions and suppressing atomic fluctuations. The doping also widens the energy gap to further reduce the NA coupling and to enhance the open-circuit voltage of perovskite solar cells. These factors increase the charge carrier lifetime by an order of magnitude and improve structural stability in the series CsPbI_1.88BrAc_0.12 > CsPbI₂Br > CsPbI₃. The fundamental atomistic insights into the influence of anion doping on the photophysical properties of the all-inorganic lead halide perovskites guide the design of efficient optoelectronic materials.

Structural Disorder in Higher-Temperature Phases Increases Charge Carrier Lifetimes in Metal Halide Perovskites

Solar cells and optoelectronic devices are exposed to heat that degrades performance. Therefore, elucidating temperature-dependent charge carrier dynamics is essential for device optimization. Charge carrier lifetimes decrease with temperature in conventional semiconductors. The opposite, anomalous trend is observed in some experiments performed with MAPbI₃ (MA = CH₃NH₃⁺) and other metal halide perovskites. Using ab initio quantum dynamics simulation, we establish the atomic mechanisms responsible for nonradiative electron-hole recombination in orthorhombic-, tetragonal-, and cubic MAPbI₃. We demonstrate that structural disorder arising from the phase transitions is as important as the disorder due to heating in the same phase. The carrier lifetimes grow both with increasing temperature in the same phase and upon transition to the higher-temperature phases. The increased lifetime is rationalized by structural disorder that induces partial charge localization, decreases nonadiabatic coupling, and shortens quantum coherence. Inelastic and elastic electron-vibrational interactions exhibit opposite dependence on temperature and phase. The partial disorder and localization arise from thermal motions of both the inorganic lattice and the organic cations and depend significantly on the phase. The structural deformations induced by thermal fluctuations and phase transitions are on the same order as deformations induced by defects, and hence, thermal disorder plays a very important role. Since charge localization increases carrier lifetimes but inhibits transport, an optimal regime maximizing carrier diffusion can be designed, depending on phase, temperature, material morphology, and device architecture. The atomistic mechanisms responsible for the enhanced carrier lifetimes at elevated temperatures provide guidelines for the design of improved solar energy and optoelectronic materials.

Dangling Octahedra Enable Edge States in 2D Lead Halide Perovskites

The structural reconstruction at the crystal layer edges of 2D lead halide perovskites (LHPs) leads to unique edge states (ES), which are manifested by prolonged carrier lifetime and reduced emission energy. These special ES can effectively enhance the optoelectronic performance of devices, but their intrinsic origin and working mechanism remain elusive. Here it is demonstrated that the ES of a family of 2D Ruddlesden-Popper LHPs [BA₂ CsPb₂ Br₇ , BA₂ MAPb₂ Br₇ , and BA₂ MA₂ Pb₃ Br₁₀ (BA = butylammonium; MA = methylammonium)] arise from the rotational symmetry elevation of the PbBr₆ octahedra dangling at the crystal layer edges. These dangling octahedra give rise to localized electronic states that enable an effective transport of electrons from the interior to layer edges, and the population of electrons in both the interior states and the ES can be manipulated via controlling the external fields. Moreover, the abundant phonons, activated by the dangling octahedra, can interact with electrons to facilitate radiative recombination, counterintuitive to the suppressive role commonly observed in conventional semiconductors. This work unveils the intrinsic atomistic and electronic origins of ES in 2D LHPs, which can stimulate the exploration of ES-based exotic optoelectronic properties and the corresponding design of high-performance devices for these emergent low-dimensional semiconductors.

Electron-Volt Fluctuation of Defect Levels in Metal Halide Perovskites on a 100 ps Time Scale

Metal halide perovskites (MHPs) have gained considerable attention due to their excellent optoelectronic performance, which is often attributed to unusual defect properties. We demonstrate that midgap defect levels can exhibit very large and slow energy fluctuations associated with anharmonic acoustic motions. Therefore, care should be taken classifying MHP defects as deep or shallow, since shallow defects may become deep and vice versa. As a consequence, charges from deep levels can escape into bands, and light absorption can be extended to longer wavelengths, improving material performance. The phenomenon, demonstrated with iodine vacancy in CH₃NH₃PbI₃ using a machine learning force field, can be expected for a variety of defects and dopants in many MHPs and other soft inorganic semiconductors. Since large-scale anharmonic motions can be precursors to chemical decomposition, a known problem with MHPs, we propose that materials that are stiffer than MHPs but softer than traditional inorganic semiconductors, such as Si and TiO₂, may simultaneously exhibit excellent performance and stability.

Perovskite-Based Memristor with 50-Fold Switchable Photosensitivity for In-Sensor Computing Neural Network

In-sensor computing can simultaneously output image information and recognition results through in-situ visual signal processing, which can greatly improve the efficiency of machine vision. However, in-sensor computing is challenging due to the requirement to controllably adjust the sensor’s photosensitivity. Herein, it is demonstrated a ternary cationic halide Cs_0.05FA_0.81MA_0.14 Pb(I_0.85Br_0.15)₃ (CsFAMA) perovskite, whose External quantum efficiency (EQE) value is above 80% in the entire visible region (400–750 nm), and peak responsibility value at 750 nm reaches 0.45 A/W. In addition, the device can achieve a 50-fold enhancement of the photoresponsibility under the same illumination by adjusting the internal ion migration and readout voltage. A proof-of-concept visually enhanced neural network system is demonstrated through the switchable photosensitivity of the perovskite sensor array, which can simultaneously optimize imaging and recognition results and improve object recognition accuracy by 17% in low-light environments.

Ag-Bi Charge Redistribution Creates Deep Traps in Defective Cs2AgBiBr6: Machine Learning Analysis of Density Functional Theory

Lead-free double perovskites hold promise for stable and environmentally benign solar cells; however, they exhibit low efficiencies because defects act as charge recombination centers. Identifying trap-assisted loss mechanisms and developing defect passivation strategies constitute an urgent goal. Applying unsupervised machine learning to density functional theory and nonadiabatic molecular dynamics, we demonstrate that negatively charged Br vacancies in Cs₂AgBiBr₆ create deep hole traps through charge redistribution between the adjacent Ag and Bi atoms. Vacancy electrons are first accepted by Bi and then shared with Ag, as the trap transforms from shallow to deep. Subsequent charge losses are promoted by Ag and Bi motions perpendicular to rather than along the Ag-Bi axis, as can be expected. In contrast, charge recombination in pristine Cs₂AgBiBr₆ correlates most with displacements of Cs atoms and Br-Br-Br angles. Doping with In to replace Ag at the vacancy maintains the electrons at Bi and keeps the trap shallow.

Collective Motion Improves the Stability and Charge Carrier Lifetime of Metal Halide Perovskites: A Phonon-Resolved Nonadiabatic Molecular Dynamics Study

By implementing a novel algorithm that realizes the constraints of certain normal modes of interest and using nonadiabatic molecular dynamics for the CsPbBr₃, we explicitly demonstrate for the first time that the collective motion between the Cs atom and inorganic octahedra facilitates to delay the nonradiative recombination of negative and positive charges. The Cs atoms can instantaneously respond to the motion of Pb and Br atoms during normal molecular dynamics, maintain the perovskite structure, and homogenize the structural distortion caused by thermal fluctuations, thus decreasing nonadiabatic coupling and charge recombination. In contrast, the perovskite becomes unstable because geometry distortion is strongly localized when the normal modes of Cs atoms are constrained, which increases the nonadiabatic coupling and accelerates charge recombination. The study emphasizes the important role of correlated motion on the stability and charge-phonon dynamics in metal halide perovskites.

Increasing Efficiency of Nonadiabatic Molecular Dynamics by Hamiltonian Interpolation with Kernel Ridge Regression

Nonadiabatic (NA) molecular dynamics (MD) goes beyond the adiabatic Born-Oppenheimer approximation to account for transitions between electronic states. Such processes are common in molecules and materials used in solar energy, optoelectronics, sensing, and many other fields. NA-MD simulations are much more expensive compared to adiabatic MD due to the need to compute excited state properties and NA couplings (NACs). Similarly, application of machine learning (ML) to NA-MD is more challenging compared with adiabatic MD. We develop an NA-MD simulation strategy in which an adiabatic MD trajectory, which can be generated with a ML force field, is used to sample excitation energies and NACs for a small fraction of geometries, while the properties for the remaining geometries are interpolated with kernel ridge regression (KRR). This ML strategy allows for one to perform NA-MD under the classical path approximation, increasing the computational efficiency by over an order of magnitude. Compared to neural networks, KRR requires little parameter tuning, saving efforts on model building. The developed strategy is demonstrated with two metal halide perovskites that exhibit complicated MD and are actively studied for various applications.

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

Lead-free metal halide perovskites are environmentally friendly and have favorable electro-optical properties; however, their efficiencies are significantly below the theoretical limit. Using ab initio nonadiabatic molecular dynamics, we show that common intrinsic defects accelerate nonradiative charge recombination in CsSnI₃ without creating midgap traps. This is in contrast to Pb-based perovskites, in which many defects have little influence on and even prolong carrier lifetimes. Sn-related defects, such as Sn vacancies and replacement of Sn with Cs are most detrimental, since Sn removal breaks the largest number of bonds and strongly perturbs the Sn-I lattice that supports the carriers. The defects increase the nonadiabatic electron-vibrational coupling and interact strongly with free carrier states. Point defects associated with I atoms are less detrimental, and therefore, CsSnI₃ synthesis should be performed in Sn rich conditions. The study provides an atomistic rationalization of why lead-free CsSnI₃ exhibits lower photovoltaic efficiency compared to some lead-based perovskites.

Interpolating Nonadiabatic Molecular Dynamics Hamiltonian with Artificial Neural Networks

Nonadiabatic (NA) molecular dynamics (MD) allows one to study far-from-equilibrium processes involving excited electronic states coupled to atomic motions. While NAMD involves expensive calculations of excitation energies and NA couplings (NACs), ground-state properties require much less effort and can be obtained with machine learning (ML) at a fraction of the ab initio cost. Application of ML to excited states and NACs is more challenging, due to costly reference methods, many states, and complex geometry dependence. We developed a NAMD methodology that avoids time extrapolation of excitation energies and NACs. Instead, under the classical path approximation that employs a precomputed ground-state trajectory, we use a small fraction (2%) of the geometries to train neural networks and obtain excited-state energies and NACs for the remaining 98% of the geometries by interpolation. Demonstrated with metal halide perovskites that exhibit complex MD, the method provides nearly two orders of computational savings while generating accurate NAMD results.

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron-phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge-charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

Why Hybrid Tin-Based Perovskites Simultaneously Improve the Structural Stability and Charge Carriers' Lifetime: Ab Initio Quantum Dynamics

Much effort has been dedicated to boost the development of lead-free perovskite solar cells. However, their performance and stability are still much less competitive to the lead-bearing counterparts. By exploiting a mixed Sn-Ge cation strategy for the development of lead-free perovskites, we perform ab initio electronic structure calculations and quantum dynamics simulations on MASn_0.5Ge_0.5I₃ and compare them to MASnI₃. The calculations demonstrate that the hybrid cation strategy can improve simultaneously the perovskite stability and the lifetime of charge carriers. The stability increases due to a larger space of possible structures within the favorable range of the structural parameters, such as the Goldschmidt tolerance and octahedron factors. By exploring the larger structure space, mixed perovskites find stable configurations with lower free energies and better fitting components that exhibit reduced fluctuations around the equilibrium geometries. Charge carriers live longer in mixed perovskites because cation mixing results in an additional and moderate disorder that separates electrons and holes, reducing their interactions while still maintaining efficient band-like charge transport. These general and fundamental principles established by the analysis of the simulation results are useful for the design of advanced materials for solar energy and construction of optoelectronic devices.

Concentric Approximation for Fast and Accurate Numerical Evaluation of Nonadiabatic Coupling with Projector Augmented-Wave Pseudopotentials

We develop an efficient and accurate method for numerical evaluation of nonadiabatic (NA) coupling in the Kohn-Sham representation with projector augmented-wave (PAW) pseudopotentials that are commonly used in electronic structure calculations on nanoscale, condensed matter, and molecular systems. Without additional cost, the method provides an order of magnitude improvement in accuracy compared to the current technique, while it is 3-4 orders of magnitude faster than the exact evaluation. Atomic displacements over typical time steps in molecular dynamics (MD) simulations are much smaller than the size of the PAW core region, and therefore, evaluation of the NA in the core is simplified. The accuracy is demonstrated with three condensed matter systems. The method is robust to variation in the MD time step. The accurate NA coupling evaluation also helps in maintaining phase-consistency of the NA coupling and identifying trivial crossings of adiabatic states. The approach stimulates NAMD applications to modeling of modern materials and processes.

Unravelling a Zigzag Pathway for Hot Carrier Collection with Graphene Electrode

The capture of photoexcited deep-band hot carriers, excited by photons with energies far above the bandgap, is of significant importance for photovoltaic and photoelectronic applications because it is directly related to the quantum efficiency of photon-to-electron conversion. By employing time-resolved photoluminescence and state-of-the-art time-domain density functional theory, we reveal that photoexcited hot carriers in organic-inorganic hybrid perovskites prefer a zigzag interfacial charge-transfer pathway, i.e., the hot carriers transfer back and forth between CH₃NH₃PbI₃ and graphene electrode, before they reach a charge-separated state. Driven by quantum coherence and interlayer vibrational modes, this pathway at the semiconductor-graphene interface takes about 400 fs, much faster than the relaxation process within CH₃NH₃PbI₃ (several picoseconds). Our work provides new insight into the fundamental understanding and precise manipulation of hot carrier dynamics at the complex interfaces, paving the way for highly efficient photovoltaic and photoelectric device optimization.

Time-Domain Ab Initio Insights into the Reduced Nonradiative Electron-Hole Recombination in ReSe2/MoS2 van der Waals Heterostructure

Two-dimensional (2D) ReSe₂ has attracted considerable interest due to its unique anisotropic mechanical, optical, and exitonic characteristics. Recent transient absorption experiments demonstrated a prolonged lifetime of photoexcited charge carriers by stacking ReSe₂ with MoS₂, but the underlying mechanism remains elusive. Here, by combining time-domain density functional theory with nonadiabatic molecular dynamics, we investigate the electronic properties and charge carrier dynamics of 2D ReSe₂/MoS₂ van der Waals (vdW) heterostructure. ReSe₂/MoS₂ has a type II band alignment that exhibits spatially distinguished conduction and valence band edges, and a built-in electric field is formed due to interface charge transfer. Remarkably, in spite of the decreased band gap and increased decoherence time, we demonstrate that the photocarrier lifetime of ReSe₂/MoS₂ is ∼5 times longer than that of ReSe₂, which originates from the greatly reduced nonadiabatic coupling that suppresses electron-hole recombination, perfectly explaining the experimental results. These findings not only provide physical insights into experiments but also shed light on future design and fabrication of functional optoelectronic devices based on 2D vdW heterostructures.

Elucidating the Influence of Sulfur Vacancies on Nonradiative Recombination Dynamics in Cu2ZnSnS4 Solar Absorbers

We report a time-domain ab initio simulation of charge carrier trapping and relaxation dynamics in pristine and defect-containing kesterite Cu₂ZnSnS₄ (CZTS) structures. Our simulations show that introduction of a neutral sulfur vacancy in the CZTS system leads to a decrease of the charge recombination rate by a factor of ∼4, and the doubly positively charged sulfur vacancy results in a minor decrease of carrier lifetime, as compared to the pristine CZTS system. The neutral sulfur vacancy weakens the nonadiabatic (NA) electron-phonon coupling by moderately localizing charge density and accelerates the pure dephasing process, extending charge carrier lifetime. Therefore, the neutral sulfur vacancy is electrically benign. The doubly positively charged sulfur vacancy introduces a subgap state which is hardly populated, and recombination of the electron and hole bypassing the trap state dominates. As a result, the recombination rate decreases in the doubly charged sulfur vacancy structure. The reported results identified the key role of the sulfur-related vacancy on charge carrier trapping and relaxation of CZTS materials, carrying important implications for further optimization of CZTS and other thin-film solar cell materials.