Showing papers in "Journal of Physical Chemistry Letters in 2023"

Understanding the Principles and Applications of Dual Z-Scheme Heterojunctions: How Far Can We Go?

Abstract: In the past seven years, dual Z-scheme heterojunctions evolved as favorable approaches for enhanced charge carrier separation through direct or indirect charge transfer transportation mechanisms. The dynamics of the charge transfer is the major strategy for understanding their photoactivity and stability through the formation of distinctive redox centers. The understanding of currently recognized principles for successful fabrication and classification in different energy and pollution remediation strategies is discussed, and a universal charge transfer-type-based classification of dual Z-schemes that can be adopted for Z-scheme and S-scheme heterojunctions is proposed. Methods used for determining the charge transfer as proof of dual Z-scheme existence are outlined. Most importantly, a new macroscopic N-scheme and a triple Z-scheme that can also be adopted as triple S-scheme heterostructures composed of four semiconductors are proposed for generating both oxidatively and reductively empowered systems. The proposed systems are expected to possess properties that enable them to harvest solar light to drive important chemical reactions for different applications.

Unveiling the Multistep Electrochemical Desorption Mechanism of Cubic NiO Films for Transmissive-to-Black Electrochromic Energy Storage Devices.

Abstract: Electrochromic smart windows offer dynamic control of sunshine and solar heat in modern architecture. Yet, how to obtain aesthetically pleasing color tuning states such as gray and black is a great challenge, and the corresponding desorption mechanism in electrochromism is still not well understood. Here, we report one transmissive-to-black NiO electrochromic film assembled by a facile and low-cost electrostatic spray technology, which achieves ultralarge optical modulation, high coloration efficiency, and remarkable energy storage capacity. By in-depth experimental analyses and the first-principle calculations, multistep electrochemical desorption mechanisms of OH- and electrochromic switching kinetics of the NiO film were unveiled. Additionally, the assembled NiO film-based smart energy storage indicator can visually display its energy storage level in real time. Our obtained NiO films and subsequent devices can serve as potential candidates in a broad range of innovative electrochromic applications including multifunctional smart windows, energy-efficient displays, energy-storage indicators, electronic labels, etc.

Chiral-Induced Spin Selectivity and Non-equilibrium Spin Accumulation in Molecules and Interfaces: A First-Principles Study.

Abstract: Electrons moving through chiral molecules are selected according to their spin orientation and the helicity of the molecule, an effect known as chiral-induced spin selectivity (CISS). The underlying physical mechanism is not yet completely understood. To help elucidate this mechanism, a non-equilibrium Green's function method, combined with a Landauer approach and density functional theory, is applied to carbon helices contacted by gold electrodes, resulting in spin polarization of transmitted electrons. Spin polarization is also observed in the non-equilibrium electronic structure of the junctions. While this spin polarization is small, its sign changes with the direction of the current and with the handedness of the molecule. While these calculations were performed with a pure exchange-correlation functional, previous studies suggest that computationally more expensive hybrid functionals may lead to considerably larger spin polarization in the electronic structure. Thus, non-equilibrium spin polarization could be a key component in understanding the CISS mechanism.

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

Abstract: 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.

Observation of a Plastic Crystal in Water-Ammonia Mixtures under High Pressure and Temperature.

Abstract: Solid mixtures of ammonia and water, the so-called ammonia hydrates, are thought to be major components of solar and extra-solar icy planets. We present here a thorough characterization of the recently reported high pressure (P)-temperature (T) phase VII of ammonia monohydrate (AMH) using Raman spectroscopy, X-ray diffraction, and quasi-elastic neutron scattering (QENS) experiments in the ranges 4-10 GPa, 450-600 K. Our results show that AMH-VII exhibits common structural features with the disordered ionico-molecular alloy (DIMA) phase, stable above 7.5 GPa at 300 K: both present a substitutional disorder of water and ammonia over the sites of a body-centered cubic lattice and are partially ionic. The two phases however markedly differ in their hydrogen dynamics, and QENS measurements show that AMH-VII is characterized by free molecular rotations around the lattice positions which are quenched in the DIMA phase. AMH-VII is thus a peculiar crystalline solid in that it combines three types of disorder: substitutional, compositional, and rotational.

Remarkable Stability and Optoelectronic Properties of an All-Inorganic CsSn0.5Ge0.5I3 Perovskite Solar Cell.

Abstract: Sn-Ge mixed perovskites have been proposed as promising lead-free candidates in the photovoltaics (PV) field. In this work, we discovered a stable P1 phase Sn-Ge mixed structure (CsSn0.5Ge0.5I3) with an appropriate band gap value of 1.19 eV, which manifests its unique structural stability and physics properties. The thermodynamic stability of this mixed structure under different growth conditions and all possible native defects are depicted in detail. The formation energies and dominant native point defects indicate that P1 phase CsSn0.5Ge0.5I3 exhibits unipolar self-doping behavior (p-type conductivity) and good defect tolerance while the growth condition changes. In addition, the calculation of light absorption confirmed that the P1 phase has a higher light absorption coefficient than that of MAPbI3 in the visible light range, showing excellent light absorption. Our work not only provides theoretical guidance for unraveling the unusual structural stability of Sn-Ge mixed perovskites, but also offers a useful scheme to modulate the stability and optoelectronic properties of Ge-based perovskites through alloy engineering.

Nanolaminated Ternary Transition Metal Carbide (MAX Phase)-Derived Core-Shell Structure Electrocatalysts for Hydrogen Evolution and Oxygen Evolution Reactions in Alkaline Electrolytes.

Abstract: The development of abundant, cheap, and highly active catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for hydrogen production. Nanolaminate ternary transition metal carbides (MAX phases) and their derived two-dimensional transition metal carbides (MXenes) have attracted considerable interest for electrocatalyst applications. Herein, four new MAX@MXene core-shell structures (Ta2CoC@Ta2CTx, Ta2NiC@Ta2CTx, Nb2CoC@Nb2CTx, and Nb2NiC@Nb2CTx), in which the core region is Co/Ni-MAX phases while the edge region is MXenes, have been prepared. Under alkaline electrolyte conditions, the Ta2CoC@Ta2CTx core-shell structure showed an overpotential of 239 mV and excellent stability during the HER with MXenes as the active sites. For the OER, the Ta2CoC@Ta2CTx core-shell structure showed an overpotential of 373 mV and a small Tafel plot (56 mV dec-1), which maintained a bulk crystalline structure and generated Co-based oxyhydroxides that formed by surface reconstruction as active sites. Considering rich chemical compositions and structures of MAX phases, this work provides a new strategy for designing multifunctional electrocatalysts and also paves the way for further development of MAX phase-based materials for clean energy applications.

Reducing Threshold of Ferroelectric Domain Switching in Ultrathin Two-Dimensional CuInP2S6 Ferroelectrics via Electrical-Mechanical Coupling.

Abstract: Room-temperature out-of-plane two-dimensional ferroelectrics have promising applications in miniaturized non-volatile memory appliances. The feasible manipulation of polarization switching significantly influences the memory performance of ferroelectrics. However, conventional high-voltage-induced polarization switching inevitably generates charge injection or electric breakdown, and large-mechanical-loading-induced polarization switching may damage the structure of ferroelectrics. Hence, decreasing critical voltage/loading for ferroelectric polarization reversal is highly required. Herein, using atomic force microscopy experiments, the ferroelectric domain switching via both electric field and mechanical loading was demonstrated for an ultrathin (∼4.1 nm) CuInP2S6 nanoflake. The relevant threshold voltage/loading for polarization switching was ∼ -5 V/1095 nN, resulting from the electric field and flexoelectric effect, respectively. Finally, the electrical-mechanical coupling was adopted to reduce the threshold voltage/loading of CuInP2S6 significantly. It can be explained by the Landau-Ginzburg-Devonshire double-well model. This effective way for easily tuning the polarization states of CuInP2S6 opens up new prospects for mechanically written and electrically erased memory devices.

Electrical Charge Coupling Dominates the Hysteresis Effect of Halide Perovskite Devices.

Abstract: Hysteresis effects in ionic-electronic devices are a valuable resource for the development of switching memory devices that can be used in information storage and brain-like computation. Halide perovskite devices show frequent hysteresis in current-voltage curves that can be harnessed to build effective memristors. These phenomena can be often described by a set of highly nonlinear differential equations that involve current, voltage, and internal state variables, in the style of the famous Hodgkin-Huxley model that accounts for the initiation and temporal response of action potentials in biological neurons. Here we extend the neuron-style models that lead to chemical inductors by introducing a capacitive coupling in the slow relaxation variable. The extended model is able to explain naturally previous observations concerning the transition from capacitor to inductor in impedance spectroscopy of MAPbBr solar cells and memristors in the dark. The model also generates new types of oscillating systems by the generation of a truly negative capacitance distinct from the usual inductive effect.

Doping Limits of Phosphorus, Arsenic, and Antimony in CdTe.

Abstract: Low p-type doping is a limiting factor to increase CdTe thin-film solar-cell efficiency toward the theoretical Shockley-Queisser limit of 33%. Previous calculations predict relatively high ionization energies for group-V acceptors (P, As, and Sb), and they are plagued by self-compensation, forming AX centers, severely limiting hole concentration. However, recent experiments on CdTe single crystals indicate a much more favorable scenario, where P, As, and Sb behave as shallow acceptors. Using hybrid functional calculations, we solve this puzzle by showing that the ionization energies significantly decrease with the supercell size. When including the effects of spin-orbit coupling and extrapolating the results to the dilute limit, we find these impurities behave as hydrogenic-like shallow acceptors, and AX centers are unstable and do not limit p-type doping. We address the differences between our results and previous theoretical predictions and show that our ionization energies predict hole concentrations that agree with recent temperature-dependent Hall measurements.

Path Topology in Molecular and Materials Sciences.

Abstract: The structures of molecules and materials determine their functions. Understanding the structure and function relationship is the holy grail of molecular and materials sciences. However, the rational design of molecules and materials with desirable functions remains a grand challenge despite decades of efforts. A major obstacle is the lack of an intrinsic mathematical characteristic that attributes to a specific function. This work introduces persistent path topology (PPT) to effectively characterize directed networks extracted from functional units, such as constitutional isomers, cis-trans isomers, chiral molecules, Jahn-Teller isomerism, and high-entropy alloy catalysts. Path homology (PH) theory is utilized to decipher the role of mirror-symmetric sublattices that hinder the formation of periodic unit cells in amorphous solids. Topological perturbation analysis (TPA) is proposed to reveal the critical target in the blood coagulation system. The proposed topological tools can be directly applied to systems biology, omics sciences, topological materials, and machine learning study of molecular and materials sciences.

Adaptive Electric Fields Embedded Electrochemical Barrier Calculations.

Abstract: Electrochemical interfaces are grand canonical ensembles of varying electrons. Simulating them by standard first-principles methods is a challenging task, since the number of electrons (or charge) is fixed in the calculation. Under the constant charge framework, we developed a constant potential simulation method realized by adding an adaptive electric field to a charge neutral system. Electric field is the controlling variable. In addition, we defined an internal reversible hydrogen electrode potential (ϕIRHE), which can ensure the model independence of our method. To validate our method, the reaction energies of some electrochemical reactions are calculated, the results are comparable with the computational hydrogen electrode model and experiments. At last, the evolution of transition state structures and charge transfer coefficients of some electrochemical reactions on Ag(111) surface were discussed by our method.

Adiabatic Trajectory Approximation: A New General Method in the Toolbox of Mixed Quantum/Classical Theory for Collisional Energy Transfer.

Abstract: A new version of the MQCT program is presented, which includes an important addition, adiabatic trajectory approximation (AT-MQCT), in which the equations of motion for the classical and quantum parts of the system are decoupled. This method is much faster, which permits calculations for larger molecular systems and at higher collision energies than was possible before. AT-MQCT is general and can be applied to any molecule + molecule inelastic scattering problem. A benchmark study is presented for H2O + H2O rotational energy transfer, an important asymmetric-top rotor + asymmetric-top rotor collision process, a very difficult problem unamenable to the treatment by other codes that exist in the community. Our results indicate that AT-MQCT represents a reliable computational tool for prediction of collisional energy transfer between the individual rotational states of two molecules, and this is valid for all combinations of state symmetries (such as para and ortho states of each collision partner).

Reversible and Irreversible Cation Intercalation in NiFeOx Oxygen Evolution Catalysts in Alkaline Media

Abstract: For electrocatalysts with a layered structure, ion intercalation is a common phenomenon. Gaining reliable information about the intercalation of ions from the electrolyte is indispensable for a better understanding of the catalytic performance of these electrocatalysts. Here, we take a holistic approach for following intercalation processes by studying the dynamics of the catalyst, water molecules, and ions during intercalation using operando soft X-ray absorption spectroscopy (XAS). Sodium and oxygen K-edge and nickel L-edge spectra were used to investigate the Na+ intercalation in a Ni0.8Fe0.2Ox electrocatalyst during the oxygen evolution reaction (OER) in NaOH (0.1 M). The Na K-edge spectra show an irreversible intensity increase upon initial potential cycling and a reversible intensity increase at the intercalation potential, 1.45 VRHE, coinciding with an increase in the Ni oxidation state. Simultaneously, the O K-edge spectra show that the Na+ intercalation does not significantly impact the hydration of the catalyst.

Conjugated Polymers in Solution: A Physical Perspective.

Abstract: Excellent progress has been made in the optoelectronic properties of conjugated polymers by controlling solution-state aggregation. However, due to the wide variety and complex structures of conjugated polymers, it is still challenging to fully understand the complex aggregation process and microstructures both in solution and in the solid state. This Perspective focuses on the chain conformations and the aggregation of conjugated polymers in solution. We discuss the factors in detail which affect solution-state aggregation and microstructures from the perspective of polymer physics in solutions, including chemical structures and environmental conditions. Based on the understanding of multiple interactions of conjugated polymers in solution, strategies to regulate solid-state microstructures and obtain high-performance polymer-based devices from solution-state aggregation are summarized.

Slower Auger Recombination in 12-Faceted Dodecahedron CsPbBr3 Nanocrystals

Abstract: Over the past two decades, intensive research efforts have been devoted to suppressions of Auger recombination in metal-chalcogenide and perovskite nanocrystals (PNCs) for the application of photovoltaics and light emitting devices (LEDs). Here, we have explored dodecahedron cesium lead bromide perovskite nanocrystals (DNCs), which show slower Auger recombination time compared to hexahedron nanocrystals (HNCs). We investigate many-body interactions that are manifested under high excitation flux density in both NCs using ultrafast spectroscopic pump–probe measurements. We demonstrate that the Auger recombination rate due to multiexciton recombinations are lower in DNCs than in HNCs. At low and intermediate excitation density, the majority of carriers recombine through biexcitonic recombination. However, at high excitation density (>1018 cm–3) a higher number of many-body Auger process dominates over biexcitonic recombination. Compared to HNCs, high PLQY and slower Auger recombinations in DNCs are likely to be significant for the fabrication of highly efficient perovskite-based photonics and LEDs.

Light-Induced Phase Segregation Evolution of All-Inorganic Mixed Halide Perovskites.

Abstract: Light-induced phase segregation in mixed halide perovskites is a major roadblock for commercialization of optoelectronics utilizing these materials. We investigate the phenomenon in a model material system consisting of only surfaces and the bulk of a single-crystalline-like microplate. We utilize environmental in-situ time-dependent photoluminescence spectroscopy to observe the bandgap evolution of phase segregation under illumination. This enables analysis of the evolution of the iodide-rich phase composition as a function of the environment (i.e., surface defects) and carrier concentration. Our study provides microscopic insights into the relationship among photocarrier generations, surface structural defects, and subsequently iodide ion migrations that result in the complex evolution of phase segregation. We elucidate the significance of surface defects with respect to the evolution of phase segregation, which may provide new perspectives for modulating ion migration by engineering of defects and carrier concentrations.

Is Unified Understanding of Vibrational Coupling of Water Possible? Hyper-Raman Measurement and Machine Learning Spectra.

Abstract: The impact of the vibrational coupling of the OH stretch mode on the spectra differs significantly between IR and Raman spectra of water. Unified understanding of the vibrational couplings is not yet achieved. By using a different class of vibrational spectroscopy, hyper-Raman (HR) spectroscopy, together with machine-learning-assisted HR spectra calculation, we examine the impact of the vibrational couplings of water through the comparison of isotopically diluted H2O and pure H2O. We found that the isotopic dilution reduces the HR bandwidths, but the impact of the vibrational coupling is smaller than in the IR and parallel-polarized Raman. Machine learning HR spectra indicate that the intermolecular coupling plays a major role in broadening the bandwidth, while the intramolecular coupling is negligibly small, which is consistent with the IR and Raman spectra. Our result clearly demonstrates a limited impact of the intramolecular vibration, independent of the selection rules of vibrational spectroscopies.

Prediction and Characterization of Two-Dimensional Zn2VN3.

Abstract: A two-dimensional (2D) monolayer of a novel ternary nitride Zn2VN3 is computationally designed, and its dynamical and thermal stability is demonstrated. A synthesis strategy is proposed based on experimental works on production of ternary nitride thin films, calculations of formation and exfoliation energies, and ab initio molecular dynamics simulations. A comprehensive characterization of 2D Zn2VN3, including investigation of its optoelectronic and mechanical properties, is conducted. It is shown that 2D Zn2VN3 is a semiconductor with an indirect band gap of 2.75 eV and a high work function of 5.27 eV. Its light absorption covers visible and ultraviolet regions. The band gap of 2D Zn2VN3 is found to be well tunable by applied strain. At the same time 2D Zn2VN3 possesses high stability against mechanical loads, point defects, and environmental impacts. Considering the unique properties found for 2D Zn2VN3, it can be used for application in optoelectronic and straintronic nanodevices.

Completely Eradicating Singlet Oxygen in Li-O2 Battery via Cobalt(II)-Porphyrin Complex-Catalyzed LiOH Chemistry.

Abstract: Li-O2 batteries have an extremely high theoretical specific energy; however, the large charge overpotential and highly reactive singlet oxygen (1O2) are two major obstacles. Porphyrin as a special kind of macrocyclic conjugated aromatic system exhibits excellent redox activity, which can be optimized by introducing a center metal atom. Herein, 5,10,15,20-tetrakis(4-aminophenyl)-porphyrin (TAPP) and 5,10,15,20-tetrakis(4-aminophenyl)-porphyrin-Co(II) (Co-TAP) are applied as effective redox mediators for Li-O2 batteries. The synergistic effects of a center metal atom and organic ligand make Co-TAP more favorable for oxygen reduction and evolution. To understand the fundamental reaction mechanisms with or without TAPP or Co-TAP, the discharge/charge processes and the parasitic reactions have been comprehensively studied. The results reveal that TAPP affects the formation mechanism of Li2O2, while Co-TAP transforms the main discharge product into LiOH without adding extra water. Co-TAP-containing batteries operated via LiOH chemistry completely eradicate 1O2 and significantly alleviate the parasitic reactions associated with 1O2.

Three-Dimensional Nanolithography with Visible Continuous Wave Laser through Triplet Up-Conversion.

Abstract: Direct laser writing (DLW) technology usually fabricates micronanostructures based on the principle of two-photon polymerization. However, two-photon polymerization requires high laser intensity which can be achieved by expensive femtosecond lasers. To address the issue, a direct laser writing method has been proposed in this work; it is based on triplet up-conversion which is characterized by its low cost, high precision, multidimensional property, and rapid processing. The feasibility of this method is jointly verified by applying both dynamic modeling and experiments. Based on the obtained results, the low laser intensity fabrication of multidimensional nanostructures is achieved. The minimum line width (∼50 nm) of micronanostructures is reached when the laser intensity is set at 2.5 × 105 W/cm2 along with a processing speed of 150 μm/s. As a result, the direct laser writing method, based on triplet up-conversion, offers a new route to achieve low-intensity and high-precision micronanostructure fabrication.

Minimal Auxiliary Basis Set Approach for the Electronic Excitation Spectra of Organic Molecules.

Abstract: We report a minimal auxiliary basis model for time-dependent density functional theory (TDDFT) with hybrid density functionals that can accurately reproduce excitation energies and absorption spectra from TDDFT while reducing cost by about 2 orders of magnitude. Our method, dubbed TDDFT-ris, employs the resolution-of-the-identity technique with just one s-type auxiliary basis function per atom for the linear response operator, where the Gaussian exponents are parametrized across the periodic table using tabulated atomic radii with a single global scaling factor. By tuning on a small test set, we determine a single functional-independent scale factor that balances errors in excitation energies and absorption spectra. Benchmarked on organic molecules and compared to standard TDDFT, TDDFT-ris has an average energy error of only 0.06 eV and yields absorption spectra in close agreement with TDDFT. Thus, TDDFT-ris enables simulation of realistic absorption spectra in large molecules that would be inaccessible from standard TDDFT.

Fast Method for Excited-State Dynamics in Complex Systems and Its Application to the Photoactivation of a Blue Light Using Flavin Photoreceptor

Abstract: The excited-state dynamics of molecules embedded in complex (bio)matrices is still a challenging goal for quantum chemical models. Hybrid QM/MM models have proven to be an effective strategy, but an optimal combination of accuracy and computational cost still has to be found. Here, we present a method which combines the accuracy of a polarizable embedding QM/MM approach with the computational efficiency of an excited-state self-consistent field method. The newly implemented method is applied to the photoactivation of the blue-light-using flavin (BLUF) domain of the AppA protein. We show that the proton-coupled electron transfer (PCET) process suggested for other BLUF proteins is still valid also for AppA.

Nanostructure Engineering and Electronic Modulation of a PtNi Alloy Catalyst for Enhanced Oxygen Reduction Electrocatalysis in Zinc-Air Batteries.

Abstract: PtNi nanoalloys have demonstrated electrocatalysis superior to that of benchmark Pt/C catalysts for the oxygen reduction reaction (ORR), yet the underlying mechanisms remain underexplored. Herein, a PtNi/NC catalyst comprising PtNi nanoparticles (∼5.2 nm in size) dispersed on N-doped carbon frameworks was prepared using a simple pyrolysis strategy. Benefiting from the individual components and a hierarchical structure, the PtNi/NC catalyst exhibited outstanding ORR activity and stability (E1/2 = 0.82 V vs RHE and 8 mV negative shift after 20000 cycles), outperforming a commercial 20 wt % Pt/C catalyst (E1/2 = 0.81 V and 32 mV negative shift). A prototype zinc-air battery constructed using PtNi/NC as the air electrode catalyst achieved highly enhanced electrochemical performance, outperforming a battery constructed using Pt/C as the ORR catalyst. Density functional theory calculations revealed that the improved ORR activity of the PtNi nanoalloys originated from charge redistribution with a suitable metal d-band center to promote the formation of the ORR intermediates.

Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series

Abstract: We report diffuse reflectivity measurements in InNbO4, ScNbO4, YNbO4, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO4 and ScNbO4 shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot–Toyozawa model. The bandgaps are 3.25 eV for CeNbO4, 4.35 eV for LaNbO4, 4.5 eV for YNbO4, and 4.73–4.93 eV for SmNbO4, EuNbO4, GdNbO4, DyNbO4, HoNbO4, and YbNbO4. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO4 to YbNbO4 and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO4, CeVO4, and LaNbO4 have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.

Machine Learning Assisted Simulations of Electrochemical Interfaces: Recent Progress and Challenges.

Abstract: The electrochemical interface, where the adsorption of reactants and electrocatalytic reactions take place, has long been a focus of attention. Some of the important processes on it tend to possess relatively slow kinetic characteristics, which are usually beyond the scope of ab initio molecular dynamics. The newly emerging technique, machine learning methods, provides an alternative approach to achieve thousands of atoms and nanosecond time scale while ensuring precision and efficiency. In this Perspective, we summarize in detail the recent progress and achievements made by the introduction of machine learning to simulate electrochemical interfaces, and focus on the limitations of current machine learning models, such as accurate descriptions of long-range electrostatic interactions and the kinetics of the electrochemical reactions occurring at the interface. Finally, we further point out the future directions for machine learning to expand in the field of electrochemical interfaces.

Atomic Model for Alkali Metal-Doped Tin-Lead Mixed Perovskites: Insight from Quantum Dynamics.

Abstract: Defects such as metal vacancies act as nonradiative recombination centers to deteriorate the photoelectric properties of metal halide perovskites. Nonadiabatic molecular dynamics has demonstrated that alkali metal dopants markedly improve the performance of mixed tin-lead perovskites. Alkali dopants increase the formation energy of tin vacancies to 1 eV, so that the defect concentration is decreased. When tin vacancies exist, alkali metals are easily doped into perovskites. Tin vacancies produce iodine trimers that create midgap states and cause rapid electron-hole recombination. Alkali metal additives eliminate the trap state, weaken nonadiabatic coupling, and decelerate charge recombination with a coefficient of ≤5.5 compared with the performance of the defective tin-lead mixed perovskite. Our research has constructed a theoretical model at the atomic level for alkali metal passivation that enhances defect tolerance of tin-lead mixed perovskites, generating valuable inspiration for optimizing high-performance perovskites.

Excitation-Wavelength-Dependent Emission Behavior in (NH4)2SnCl6 via Sb3+ Dopant

Abstract: With high photoluminescence efficiency and a simple solution synthesis method, lead halide perovskites are expected to be a promising material for display and illumination. However, the toxicity and environmental sensitivity of lead hinder its potential applications. Here, we introduced Sb3+ ions into the lead-free perovskites derivative (NH4)2SnCl6 via a doping strategy. For the first time we synthesis the excitation-dependent perovskite with dynamically tunable fluorescence from yellow to near-infrared (NIR) emission by varying the UV excitation from 360 to 390 nm at room temperature. The DFT calculations are highly consistent no matter whether the coordination number of Sb3+ is 5 or 6. In contrasting to the early report of Sb triplet emission in the Sb doped perovskite, this material give a mixed self-trapped exciton (STE) emission. The 590 nm emission band is derived from the STE of SbCl5, and the 734 nm NIR emission band is attributed to the Sb–Sn mixed STE, which is supported by DFT calculations and spectral results. This study provides guidance for the design of perovskite phosphors with high efficiency and excitation-dependent properties.

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

Abstract: 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.

Rational Selection of the Lewis Base Molecules Targeted for Lead-Based Defects of Perovskite Solar Cells: The Synergetic Co-passivation of Carbonyl and Carboxyl Groups.

Abstract: Defect passivation through Lewis acid-base chemistry has recently attracted significant interest because of its proven ability to improve the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). However, tedious trial-and-error procedures are commonly used for the selection of Lewis molecules due to their abundant variety. Herein, two typical Lewis base molecules, the M molecule containing only carbonyl groups and the 3M molecule containing both carbonyl and carboxyl groups, are proposed to passivate the Pb-based defects and mitigate their negative impacts on PSC performance. The results indicated that much stronger coordination bonds can be formed between the 3M molecule and uncoordinated Pb2+ than with the M molecule. Because of the benefit from the synergetic co-passivation effect of carbonyl and carboxyl groups, an impressive maximum PCE of 24.07% was achieved via 3M modification. More importantly, the modified devices demonstrated remarkably improved operational stability.