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

Decoherence ensures convergence of non-adiabatic molecular dynamics with number of states

Non-adiabatic (NA) molecular dynamics (MD) is a powerful approach for studying far-from-equilibrium quantum dynamics in photophysical and photochemical systems. Most NA-MD methods are developed and tested with few-state models, and their validity with complex systems involving many states is not well studied. By modeling intraband equilibration and interband recombination of charge carriers in MoS2, we investigate the convergence of three popular NA-MD algorithms, fewest switches surface hopping (FSSH), global flux surface hopping (GFSH), and decoherence induced surface hopping (DISH) with the number of states. Only the standard DISH algorithm converges with the number of states and produces Boltzmann equilibrium. Unitary propagation of the wave function in FSSH and GFSH violates the Boltzmann distribution, leads to internal inconsistency between time-dependent Schrödinger equation state populations and trajectory counts, and produces non-convergent results. Introducing decoherence in FSSH and GFSH by collapsing the wave function fixes these problems. The simplified version of DISH that omits projecting out the occupied state and is applicable to few-state systems also causes problems when the number of states is increased. We discuss the algorithmic application of wave function collapse and Boltzmann detailed balance and provide detailed FSSH, GFSH, and DISH flow charts. The use of convergent NA-MD methods is highly important for modeling complicated quantum processes involving multiple states. Our findings provide the basis for investigating quantum dynamics in realistic complex systems.

First-principles study of phase-dependent carrier transport mechanism for MASnI3Sn-based halide perovskite

Organic-inorganic hybrid perovskites have attracted tremendous attentions owing to their excellent properties as next-generation photovoltaic devices. With soft covalent framework, organic-inorganic hybrid perovskites exhibit different phases at different temperatures. The band-edge features of perovskites are mainly contributed by inorganic framework, which means the structural differences between these phases would lead to complex carrier transport. We investigated the carrier transport of Sn-based organic-inorganic hybrid perovskite CH3NH3SnI3(MASnI3), considering acoustic deformation potential scattering, ionized impurity scattering, and polar optical phonon scattering. It is found that the electron mobility of each phase of MASnI3is strongly correlated with the Sn-I-Sn bond angle and there is in-plane/out-of-plane anisotropy. The projected crystal orbital Hamilton population analysis suggested that the tilt and rotation of the [SnI6]4-octahedron influence the Sn(p)-I(p) orbital electron coupling and the electron transport, leading to different band-edge features in multiple phases. The carrier mobility with respect to temperature was further calculated for each phase of MASnI3in respective temperature intervals, showing lower carrier mobility in high temperature. Comparing the contribution of different scattering mechanisms, it was found that the dominant scattering mechanism is polar optical phonon scattering, while multiple scattering mechanisms compete in individual cases.

Nonadiabatic Molecular Dynamics in Momentum Space Beyond Harmonic Approximation: Hot Electron Relaxation in Photoexcited Black Phosphorus

We simulated hot-electron relaxation in black phosphorus using the nonadiabatic molecular dynamics (NA-MD) approach with a non-Condon effect in momentum space beyond the harmonic approximation. By comparing simulations at the Γ point in a large supercell with those using a few k-points in a smaller supercell─while maintaining the same number of electronic states within the same energy range, we demonstrate that both setups yield remarkably consistent energy relaxation times, regardless of the initial state energy. This consistency arises from the complementary effects of supercell size in real space and the number of k-points in the reciprocal space. This finding confirms that simulations at a single k-point in large size supercells are an effective approximation for NA-MD with a non-Condon effect. This approach offers significant advantages for complex photophysics, such as intervalley scattering and indirect bandgap charge recombination, and is particularly suitable for large systems without the need for a harmonic approximation.

Unveiling the Effect of Cooling Rate on Grown-in Defects Concentration in Polycrystalline Perovskite Films for Solar Cells with Improved Stability

Numerous efforts are devoted to reducing the defects at perovskite surface and/or grain boundary; however, the grown-in defects inside grain is rarely studied. Here, the influence of cooling rate on the point defects concentration in polycrystalline perovskite film during heat treatment processing is investigated. With the combination of theoretical and experimental studies, this work reveals that the supersaturated point defects in perovskite films generate during the cooling process and its concentration improves as the cooling rate increases. The supersaturated point defects can be minimized through slowing the cooling rate. As a result, the optimized FAPbI3 polycrystalline films achieve a superior carrier lifetime of up to 12.6 µs and improved stability. The champion device delivers a 25.47% PCE (certified 24.7%) and retain 90% of their initial value after >1100 h of operation at the maximum power point. These results provide a fundamental understanding of the mechanisms of grown-in defects formation in polycrystalline perovskite film.

Eliminating Non-Corner-Sharing Octahedral for Efficient and Stable Perovskite Solar Cells

The metal halide (BX6)4- octahedron, where B represents a metal cation and X represents a halide anion, is regarded as the fundamental structural and functional unit of metal halide perovskites. However, the influence of the way the (BX6)4- octahedra connect to each other has on the structural stability and optoelectronic properties of metal halide perovskite is still unclear. Here, the octahedral connectivity, including corner-, edge-, and face-sharing, of various CsxFA1-xPbI3 (0 ≤ x ≤ 0.3) perovskite films is tuned and reliably characterized through compositional and additive engineering, and with ultralow-dose transmission electron microscopy. It is found that the overall solar cell device performance, the charge carrier lifetime, the open-circuit voltage, and the current density-voltage hysteresis are all improved when the films consist of corner-sharing octahedra, and non-corner sharing phases are suppressed, even in films with the same chemical composition. Additionally, it is found that the structural, optoelectronic, and device performance stabilities are similarly enhanced when non-corner-sharing connectivities are suppressed. This approach, combining macroscopic device tests and microscopic material characterization, provides a powerful tool enabling a thorough understanding of the impact of octahedral connectivity on device performance, and opens a new parameter space for designing high-performance photovoltaic metal halide perovskite devices.

Compression of Organic Molecules Coupled with Hydrogen Bonding Extends the Charge Carrier Lifetime in BA2SnI4

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4 (BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron-hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron-hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn-I bonds, distorts the [SnI6]4- octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn-I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron-hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

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.

Impact of large A-site cations on electron-vibrational interactions in 2D halide perovskites: Ab initio quantum dynamics

Using ab initio nonadiabatic molecular dynamics, we study the effect of large A-site cations on nonradiative electron-hole recombination in two-dimensional Ruddlesden-Popper perovskites HA2APb2I7, HA = n-hexylammonium, A = methylammonium (MA), or guanidinium (GA). The steric hindrance created by large GA cations distorts and stiffens the inorganic Pb-I lattice, reduces thermal structural fluctuations, and maintains the delocalization of electrons and holes at ambient and elevated temperatures. The delocalized charges interact more strongly in the GA system than in the MA system, and the charge recombination is accelerated. In contrast, replacement of only some MA cations with GA enhances disorder and increases charge lifetime, as seen in three-dimensional perovskites. This study highlights the key influence of structural fluctuations and disorder on the properties of charge carriers in metal halide perovskites, providing guidance for tuning materials' optoelectronic performance.

Nuclear Quantum Effects Accelerate Charge Recombination but Boost the Stability of Inorganic Perovskites in Mild Humidity

Experiments have demonstrated that mild humidity can enhance the stability of the CsPbBr3 perovskite, though the underlying mechanism remains unclear. Utilizing ab initio molecular dynamics, ring polymer molecular dynamics, and non-adiabatic molecular dynamics, our study reveals that nuclear quantum effects (NQEs) play a crucial role in stabilizing the lattice rigidity of the perovskite while simultaneously shortening the charge carrier lifetime. NQEs reduce the extent of geometric disorder and the number of atomic fluctuations, diminish the extent of hole localization, and thereby improve the electron-hole overlap and non-adiabatic coupling. Concurrently, these effects significantly suppress phonon modes and slow decoherence. As a result, these factors collectively accelerate charge recombination by a factor of 1.42 compared to that in scenarios excluding NQEs. The resulting sub-10 ns recombination time scales align remarkably well with experimental findings. This research offers novel insight into how moisture resistance impacts the stability and charge carrier lifetime in all-inorganic perovskites.

Understanding the Behaviors of Plasmon-Induced Hot Carriers and Their Applications in Photocatalysis

Photocatalysis driven by plasmon-induced hot carriers has been gaining increasing attention. Recent studies have demonstrated that plasmon-induced hot carriers can directly participate in photocatalytic reactions, leading to great enhancement in solar energy conversion efficiency, by improving the catalytic activity or changing selectivity. Nevertheless, the utilization efficiency of hot carriers remains unsatisfactory. Therefore, how to correctly understand the generation and transfer process of hot carriers, as well as accurately differentiate between the possible mechanisms, have become a key point of attention. In this review, we overview the fundamental processes and mechanisms underlying hot carrier generation and transport, followed by highlighting the importance of hot carrier monitoring methods and related photocatalytic reactions. Furthermore, possible strategies for the further characterization of plasmon-induced hot carriers and boosting their utilization efficiency have been proposed. We hope that a comprehensive understanding of the fundamental behaviors of hot carriers can aid in designing more efficient photocatalysts for plasmon-induced photocatalytic reactions.

Photocatalytic ethylene production by oxidative dehydrogenation of ethane with dioxygen on ZnO-supported PdZn intermetallic nanoparticles

The selective oxidative dehydrogenation of ethane (ODHE) is attracting increasing attention as a method for ethylene production. Typically, thermocatalysts operating at high temperatures are needed for C-H activation in ethane. In this study, we describe a low temperature ( < 140 °C) photocatalytic route for ODHE, using O2 as the oxidant. A photocatalyst containing PdZn intermetallic nanoparticles supported on ZnO is prepared, affording an ethylene production rate of 46.4 mmol g-1 h-1 with 92.6% ethylene selectivity under 365 nm irradiation. When we employ a simulated shale gas feed, the photocatalytic ODHE system achieves nearly 20% ethane conversion while maintaining an ethylene selectivity of about 87%. The robust interface between the PdZn intermetallic nanoparticles and ZnO support plays a crucial role in ethane activation through a photo-assisted Mars-van Krevelen mechanism, followed by a rapid lattice oxygen replenishment to complete the reaction cycle. Our findings demonstrate that photocatalytic ODHE is a promising method for alkane-to-alkene conversions under mild conditions.

Nonadiabatic Molecular Dynamics with Non-Condon Effect of Charge Carrier Dynamics

Nonradiative multiphonon transitions play a crucial role in understanding charge carrier dynamics. To capture the non-Condon effect in nonadiabatic molecular dynamics (NA-MD), we develop a simple and accurate method to calculate noncrossing and crossing k-point NA coupling in momentum space on an equal footing and implement it with a trajectory surface hopping algorithm. Multiple k-point MD trajectories can provide sufficient nonzero momentum multiphonons coupled to electrons, and the momentum conservation is maintained during nonvertical electron transition. The simulations of indirect bandgap transition in silicon and intra- and intervalley transitions in graphene show that incorporation of the non-Condon effect is needed to correctly depict these types of charge dynamics. In particular, a hidden process is responsible for the delayed nonradiative electron-hole recombination in silicon: the thermal-assisted rapid trapping of an excited electron at the conduction band minimum by a long-lived higher energy state through a nonvertical transition extends charge carrier lifetime, approaching 1 ns, which is about 1.5 times slower than the direct bandgap recombination. For graphene, intervalley scattering takes place within about 225 fs, which can occur only when the intravalley relaxation proceeds to about 50 fs to gain enough phonon momentum. The intra- and intervalley scattering constitute energy relaxation, which completes within sub-500 fs. All the simulated time scales are in excellent agreement with experiments. The study establishes the underlying mechanisms for a long-lived charge carrier in silicon and valley scattering in graphene and underscores the robustness of the non-Condon approximation NA-MD method, which is suitable for rigid, soft, and large defective systems.

Blocking recombination centers by controlling the charge density of a sulfur vacancy in antimony trisulfide

By performing nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we have explored the effects of the charge density of a sulfur vacancy on charge trapping and recombination in antimony trisulfide (Sb2S3). The simulations demonstrate that, compared to an antimony vacancy, the sulfur vacancy generates a high charge density trap state within the band gap. This state acts as the recombination center and provides new channels for charge carrier relaxation. Filling the sulfur vacancy with electron donors elevates the defect state to the Fermi level due to the introduced extra electrons. In contrast, the electron acceptor lowers the charge density of the sulfur vacancy by capturing its local electrons, eliminating the charge recombination center and extending the photo-generated charge carrier lifetime. Additionally, compared with electron injection, hole injection can also decrease the charge density of the trap state via neutralizing its local electronic states, eliminate the trap state within the band gap, and suppress nonradiative electron-hole recombination. This study is expected to shed new light on the blocking recombination centers and provide valuable insights into the design of high-performance solar cells.

Lattice Distortion and Low-Frequency Anharmonic Phonons Suppress Charge Recombination in Lead Halide Perovskites upon Pseudohalide Doping: Time-Domain Ab Initio Analysis

Perovskite solar cells have witnessed a surge in interest as a promising technology for low-cost, high-efficiency photovoltaics with certified power conversion efficiencies beyond 25%. However, their commercial development is hindered by poor stability and nonradiative losses that restrict their approach to the theoretical efficiency limit. Using ab initio nonadiabatic molecular dynamics, we demonstrate that nonradiative charge recombination is suppressed when the iodide in formamidinium lead iodide (FAPbI3) is partially replaced with pseudohalide anions (SCN-, BF4-, and PF6-). The replacement breaks the symmetry of the system and creates local structural distortion and dynamic disorder, decreasing electron-hole overlap and nonadiabatic electron-vibrational coupling. The charge carrier lifetime is found to increase with increased structural distortion and is the longest for PF6-. This work is fundamentally relevant to the design of high-performance perovskite materials for optoelectronic applications.

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.

Lessons Learned from Catalysis to Qubits: General Strategies to Build Accessible and Accurate First-Principles Models of Point Defects

Defects and dopants play critical roles in defining the properties of a material. Achieving a mechanistic understanding of how such properties arise is challenging with current experimental methods, and computational approaches suffer from significant modeling limitations that frequently require a posteriori fitting. Consequently, the pace of dopant discovery as a means of tuning material properties for a particular application has been slow. However, recent advances in computation have enabled researchers to move away from semiempirical schemes to reposition density functional theory as a predictive tool and improve the accessibility of highly accurate first-principles methods to all researchers. This Perspective discusses some of these recent achievements that provide more accurate first-principles geometric, thermodynamic, optical, and electronic properties simultaneously. Advancements related to supercells, basis sets, functionals, and optimization protocols, as well as suggestions for evaluating the quality of a computational model through comparison to experimental data, are discussed. Moreover, recent computational results in the fields of energy materials, heterogeneous catalysis, and quantum informatics are reviewed along with an evaluation of current frontiers and opportunities in the field of computational materials chemistry.

Exploring the effect of C6H5-x/FxBr (x = 0-3) passivating agent on surface properties at different termination ends: first principles

To prevent further decomposition of organic-inorganic hybrid perovskite by defects, in this work density functional theory was applied to explore the electronic properties, carrier surface mobility and theoretical photoelectric conversion efficiency (PCE) of passivating molecules with different fluorine atom content at the symmetric site of the benzene ring at different termination ends of MAPbI3, which shed light on the control of perovskite surface passivation by different element atoms in the same molecule. We found that the same molecule acts as a different passivation agent at different termination faces. Passivating molecules on the surface termination end by MAI play a Lewis acid role, with molecules with stronger dipole moments narrowing the band gap from the original 1.77 to 1.73 eV. The exciton binding energy of molecules with stronger dipole moments (0.187-0.292 meV) is significantly lower than that of MAPbI3 (0.332 meV), so the effective separation of interface electrons and holes can be realized. Bromopenta-fluorobenzene has a lower adsorption energy of -0.17 eV, which can stably adsorb on the surface of perovskite and increase visible light absorption. Ultimately, the theoretical PCE increased from 15.8% to 16.16%. In addition, on the surface terminated by PbI2, BrB with a strong dipole moment can provide electrons for Pb2+ and act as a Lewis base. At the surface end, it can form an ionic bond with Pb2+, while the antibonding molecular orbital characteristic is dominant, which increases the band gap from 1.76 to 1.87 eV. After increasing to 4-F-BrB, the fluorine atom has strong electronegativity and can easily bond with Pb2+. The conjugate π cycle intensifies the promotion of electron transfer, reducing the work function from 5.262 to 4.703 eV, reducing the effective electron and hole mass (0.514, 0.204 m0), and improving the photovoltaic performance. Finally, increasing the number of passivation molecules resulted in a decrease in the PCE from 15.93% to 14.75%.

Mn-Modified ZnO Nanoflakes for Optimal Photoelectrochemical Performance Under Visible Light: Experimental Design and Theoretical Rationalization

Doping of zinc oxide (ZnO) with manganese (Mn) tunes midbandgap states of ZnO to enhance its optical properties and makes it into an efficient photoactive material for photoelectrochemical water splitting, waste removal from water, and other applications. We demonstrate that ZnO modified with 1 at. % Mn exhibits the best performance, as rationalized by experimental, structural, and optical characterization and theoretical analysis. ZnO doped with the optimal Mn content possesses improved light absorption in the visible region and minimizes charge carrier recombination. The doping is substitutional and creates midgap states near the valence band. Mn atoms break localized charge traps at oxygen vacancy sites and eliminate photoluminescence peaks associated with oxygen vacancies. The optimal performance of Mn-modified ZnO is demonstrated with the photodegradation of Congo red and photoelectrochemical water splitting.

TiO2 nanorod/nanotube interface reconstruction and synergistic role of oxygen vacancies and gold in H-Au-TiO2 NR/NT for photoelectrochemical bacterial inactivation and water splitting

Photoelectrochemical (PEC) water splitting by semiconductor photoanodes is limited by sluggish water oxidation kinetics coupled with serious charge recombinations. In this paper, an effective strategy of TiO2 nanorod/nanotube nanostructured interface reconstruction, oxygen vacancies and surface modification were employed for stability and efficient charge transport in the photoanodes. Successive anodization and hydrothermal routes were adopted for the TiO2 NR/NT photoanodes interface reconstruction, followed by Au nanoparticles/clusters (Au NP) loading and hydrogen treatment. This resulted in H-Au-TiO2 NR/NT photoanodes. A three-dimensional structure of TiO2 NR on TiO2 NT/Ti foil nanotubes achieved the highest photocurrent density (1.42 mA cm-2 at 0.3 V vs. Ag/AgCl). The optimal oxygen vacancies and Au NP loading on TiO2 NR/NT exhibited 1.62 mA cm-2 photocurrent density at 0.3 V vs. Ag/AgCl in H-Au-TiO2 NR/NT photoelectrode, which is eight times higher than the TiO2 NT/Ti foil. TRPL analyses confirm the hydrogen treatments to TiO2 exhibited the emission lifetime (46 ns) in the H-Au-TiO2 NR/NT photoanodes due to newly formed lower Ti3+-related trapped electron states and Au NP. The optimum H-Au (4)-TiO2 NR/NT photoanodes achieved 95% photoelectrochemical (PEC) bacterial inactivation and effective PEC water splitting with (278 and 135.4) μmol of hydrogen and oxygen generation, respectively. In this study, oxygen vacancies combined with gold particles and interface reconstruction provide an innovative way to design effective photoelectrodes.

Twist Angle-Dependent Intervalley Charge Carrier Transfer and Recombination in Bilayer WS2

A twist angle at a van der Waals junction provides a handle to tune its optoelectronic properties for a variety of applications, and a comprehensive understanding of how the twist modulates electronic structure, interlayer coupling, and carrier dynamics is needed. We employ time-dependent density functional theory and nonadiabatic molecular dynamics to elucidate angle-dependent intervalley carrier transfer and recombination in bilayer WS2. Repulsion between S atoms in twisted configurations weakens interlayer coupling, increases the interlayer distance, and softens layer breathing modes. Twisting has a minor influence on K valleys while it lowers Γ valleys and raises Q valleys because their wave functions are delocalized between layers. Consequently, the reduced energy gaps between the K and Γ valleys accelerate the hole transfer in the twisted structures. Intervalley electron transfer proceeds nearly an order of magnitude faster than hole transfer. The more localized wave functions at K than Q values and larger bandgaps result in smaller nonadiabatic couplings for intervalley recombination, making it 3-4 times slower in twisted than high-symmetry structures. B2g breathing, E2g in-plane, and A1g out-of-plane modes are most active during intervalley carrier transfer and recombination. The faster intervalley transfer and extended carrier lifetimes in twisted junctions are favorable for optoelectronic device performance.

Surface Hydroxylation during Water Splitting Promotes the Photoactivity of BiVO4(010) Surface by Suppressing Polaron-Mediated Charge Recombination

Polaron-based electron transport restricts the photoelectrochemical (PEC) water splitting efficiency of BiVO4. However, the location and dynamics of polarons are significantly dependent on the surface hydroxylation. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrated that hydroxylation of BiVO4(010) surface greatly alleviates the detrimental effect of oxygen-vacancy-induced electron polaron (EP). Surface hydroxylation stabilizes the EP at the surface to facilitate water splitting, makes the polaron a shallow localized state, and reduces the intensity of high-frequency V-O bond stretching vibrations. By decreasing the nonadiabatic coupling and decoherence time, the charge carrier lifetimes are extended by 1-3 orders of magnitude depending on the hydroxylation coverage. Our study not only reveals that the surface hydroxylation mitigated detrimental impacts of polarons in metal oxides but also provided valuable insights into the benign effect of intermediate species on the photocatalytic reactivity.

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.

Zeno and Anti-Zeno Effects in Nonadiabatic Molecular Dynamics

Decoherence plays an important role in nonadiabatic (NA) molecular dynamics (MD) simulations because it provides a physical mechanism for trajectory hopping and can alter transition rates by orders of magnitude. Generally, decoherence effects slow quantum transitions, as exemplified by the quantum Zeno effect: in the limit of infinitely fast decoherence, the transitions stop. If the measurements are not sufficiently frequent, an opposite quantum anti-Zeno effect occurs, in which the transitions are accelerated with faster decoherence. Using two common NA-MD approaches, fewest switches surface hopping and decoherence-induced surface hopping, combined with analytic examination, we demonstrate that including decoherence into NA-MD slows down NA transitions; however, many realistic systems operate in the anti-Zeno regime. Therefore, it is important that NA-MD methods describe both Zeno and anti-Zeno effects. Numerical simulations of charge trapping and relaxation in graphitic carbon nitride suggest that time-dependent NA Hamiltonians encountered in realistic systems produce robust results with respect to errors in the decoherence time, a favorable feature for NA-MD simulations.

Interpolating Nonadiabatic Molecular Dynamics Hamiltonian with Bidirectional Long Short-Term Memory Networks

Essential for understanding far-from-equilibrium processes, nonadiabatic (NA) molecular dynamics (MD) requires expensive calculations of the excitation energies and NA couplings. Machine learning (ML) can simplify computation; however, the NA Hamiltonian requires complex ML models due to its intricate relationship to atomic geometry. Working directly in the time domain, we employ bidirectional long short-term memory networks (Bi-LSTM) to interpolate the Hamiltonian. Applying this multiscale approach to three metal-halide perovskite systems, we achieve two orders of magnitude computational savings compared to direct ab initio calculation. Reasonable charge trapping and recombination times are obtained with NA Hamiltonian sampling every half a picosecond. The Bi-LSTM-NAMD method outperforms earlier models and captures both slow and fast time scales. In combination with ML force fields, the methodology extends NAMD simulation times from picoseconds to nanoseconds, comparable to charge carrier lifetimes in many materials. Nanosecond sampling is particularly important in systems containing defects, boundaries, interfaces, etc. that can undergo slow rearrangements.

Spin-Orbit Coupling Notably Retards Non-radiative Electron-Hole Recombination in Methylammonium Lead Triiodide Perovskites

The giant spin-orbit coupling (SOC) of a heavy lead element significantly extends charge carrier lifetimes of lead halide perovskites (LHPs). The physical mechanism remains unclear and requires a quantum dynamics perspective. Taking methylammonium lead iodide (MAPbI₃) as a prototypical system and using non-adiabatic molecular dynamics combined with 1/2 electron correction, we show that SOC notably reduces the non-radiative electron-hole (e-h) recombination by decreasing the non-adiabatic coupling (NAC) primarily as a result of SOC decreasing the e-h wave function overlap by reshaping the electron and hole wave functions. Second, SOC causes spin mismatch subject to spin-mixed states, which further decreases NAC. The charge carrier lifetime is about 3-fold longer in the present of SOC relative to the absence of SOC. Our study generates the fundamental understanding of SOC minimizing non-radiative charge and energy losses in LHPs.

Excitation-Wavelength-Dependent Charge-Carrier Lifetime in Hematite: An Insight from Nonadiabatic Molecular Dynamics

Experiments have reported that the photoexcited carrier lifetime in α-Fe₂O₃ has a significant excitation-wavelength dependence but leave the physical mechanism unresolved. In this work, we rationalize the puzzling excitation-wavelength dependence of the photoexcited carrier dynamics in Fe₂O₃ by performing nonadiabatic molecular dynamics simulation based on the strongly constrained and appropriately normed functional, which accurately describes the electronic structure of Fe₂O₃. Photogenerated electrons with lower-energy excitation relax fast in the t_2g conduction band within about 100 fs, while the photogenerated electrons with higher-energy excitation undergo first a slower interband relaxation from the e_g lower state to the t_2g upper state on a time scale of 135 ps, followed by the much faster t_2g intraband relaxation. This study provides insight into the experimentally reported excitation-wavelength dependence of the carrier lifetime in Fe₂O₃ and a reference for regulating photogenerated carrier dynamics in transition-metal oxides through the light excitation wavelength.

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.

Tuning Octahedral Tilting by Doping to Prevent Detrimental Phase Transition and Extend Carrier Lifetime in Organometallic Perovskites

As one of the most promising materials for next-generation solar cells, organometallic perovskites have attracted substantial fundamental and applied interest. Using first-principles quantum dynamics calculations, we show that octahedral tilting plays an important role in stabilizing perovskite structures and extending carrier lifetimes. Doping the material with (K, Rb, Cs) ions at the A-site enhances octahedral tilting and the stability of the system relative to unfavorable phases. The stability of doped perovskites is maximized for uniform distribution of the dopants. Conversely, aggregation of dopants in the system inhibits octahedral tilting and the associated stabilization. The simulations also indicate that with enhanced octahedral tilting, the fundamental band gap increases, the coherence time and nonadiabatic coupling decrease, and the carrier lifetimes are thus extended. Our theoretical work uncovers and quantifies the heteroatom-doping stabilization mechanisms, opening up new avenues to enhancing the optical performance of organometallic perovskites.

Passivation of Hematite by a Semiconducting Overlayer Reduces Charge Recombination: An Insight from Nonadiabatic Molecular Dynamics

Hematite (α-Fe₂O₃) is a promising photoanode material for photoelectrochemical water splitting. Surface-passivating layers are effective in improving water oxidation kinetics; however, the passivation mechanism is not fully understood due to the complexity of interfacial reactions. Focusing on the Fe-terminated Fe₂O₃ (0001) surface that exhibits surface states in the band gap, we perform ab initio quantum dynamics simulations to study the effect of an α-Ga₂O₃ overlayer on charge recombination. The overlayer eliminates surface states and suppresses charge recombination 4-fold. This explains in part the observed cathodic shift in the onset potential for water oxidation. The increased charge carrier lifetime is an outcome of two factors, energy gap and electron-vibrational coupling, with a positive contribution from the former but a negative contribution from the latter. This work presents an advance in the atomistic time-domain understanding of the influence of surface passivation on charge recombination dynamics and provides guidance for designing novel α-Fe₂O₃ photoanodes.

Superior Limit of Light-Absorption Improvement in Two-Dimensional Haeckelite GaN-ZnO by Nonadiabatic Molecular Dynamics Simulation

A weak internal electrostatic field is usually required to improve optical performance; however, this is not the case in two-dimensional haeckelite (8|4) GaN-ZnO that has physical properties that are better than those of their binary counterparts. By performing nonadiabatic molecular dynamics simulations, we ascribe the superior limit of improvement of light absorption to the convergence of the electron-hole recombination time when the thickness of the 8|4 phase exceeds a critical value, which arises from the competition between nonadiabatic coupling and quantum decoherence. We show that nonadiabatic coupling continuously becomes weaker because of the reduced nucleus velocity with an increase in thickness. We further demonstrate that the quantum decoherence is first accelerated and then decelerated because of the thickness-dependent electron-phonon coupling controlled by the peculiar in-plane A' and A″ phonon modes. Our study clarifies the issue with regard to light absorption, which provides useful guidance for improving our understanding of the optical properties in two-dimensional polar semiconductors.

Design, synthesis, and characterization of a new hybrid organic-inorganic perovskite with a high-Tc dielectric transition

Phase change materials (PCMs) have drawn increasing attention for their promising applications in thermal switches, data communication, and energy storage. Because of the complexity of the interactions between molecules, it is still a challenge to design PCMs with a desired high phase transition temperature (T_c). In this study, a one-dimensional hybrid perovskite of (TEACCl)PbBr₃ (1, TEACCl = Et₃NCH₂Cl) was successfully designed and synthesized with a T_c = 390 K. Disordering of TEACCl⁺ on the heating process is the origin of the structural phase transition of 1 from the P2₁/c to P6₃/mmc structure. It is noted that the phase transition is associated with an excellent switchable dielectric property, which indicates that 1 has the potential to be applied to sensor equipment. After calculation, 1 is an infrequent indirect bandgap semiconductor with an energy gap of 3.57 eV. Moreover, 1 exhibits strong red fluorescence under irradiation of UV light. This work will provide guidance for designing high T_c switching materials.

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.

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.