Charge Dynamics Research Articles

Using the Haken-Strobl-Reineker Model to Determine the Temperature Dependence of the Diffusion Coefficient.

Stochastic quantum Liouville equations (SQLE) are widely used to model energy and charge dynamics in molecular systems. The Haken-Strobl-Reineker (HSR) SQLE is a particular paradigm in which the dynamical noise that destroys quantum coherences arises from a white noise (i.e., constant-frequency) spectrum. A system subject to the HSR SQLE thus evolves to its "high-temperature" limit, whereby all the eigenstates are equally populated. This result would seem to imply that the predictions of the HSR model, e.g., the temperature dependence of the diffusion coefficient, have no validity for temperatures lower than the particle bandwidth. The purpose of this paper is to show that this assumption is incorrect for translationally invariant systems. In particular, provided that the diffusion coefficient is determined via the mean-squared-displacement, considerations about detailed-balance are irrelevant. Consequently, the high-temperature prediction for the temperature dependence of the diffusion coefficient may be extrapolated to lower temperatures, provided that the bath remains classical. Thus, for diagonal dynamical disorder the long-time diffusion coefficient, D∞(T) = c1/T, while for both diagonal and off-diagonal disorder, D∞(T) = c1/T + c2T, where c2 ≪ c1. An appendix discusses an alternative interpretation from the HSR model of the "quantum to classical" dynamics transition, whereby the dynamics is described as stochastically punctuated coherent motion.

Optimized SnO2 preparation and coherent interlayers for enhanced charge dynamics and efficient perovskite photovoltaics

The dense, uniform and conformal electron transport layers (ETLs) will largely promote charge separation and extraction. Here, the mixed acid (hydrochloric acid and nitric acid) was used to regulate preparation process and enhance utilization of materials, and the colloids of tin oxide (SnO2) nanocrystals were prepared through hydrothermal process. The complete dissolution of Sn source can increase purity, produce homogeneous precursor, reduce grain sizes and improve film-coverage. As confirmed, a coherent interlayer at the SnO2 ETLs/perovskite interfaces will be achieved by coupling a Cl-bonded SnO2 film with a Cl-containing perovskite precursor. This thin coherent interlayer will largely reduce interface traps, enhance rapid carrier extraction, and impede charge recombination. The uniform polymer phase of (PEO)120-(PPO)30 will be used to passivate traps at the grain boundaries of perovskite films and further improve the photovoltaic performance. The maximum energy conversion efficiency (23.17%, a VOC of 1.153 V, a JSC of 24.75 mA cm−2 and a FF of 0.812) of perovskite solar cells was achieved. The charge separation, extraction, and recombination kinetics (charge dynamic process) was determined by the related characterization techniques. The functionalized SnO2-ETLs and formed coherent interlayer will provide a simple strategy to effectively decrease interface traps, enhance charge extraction, and facilitate development of perovskite solar cells.

Exploiting Mechanism of Enhanced Charge Transfer in Ternary Organic Solar Cells at Low Energy Loss

The structural disorder and aggregation of the third acceptor with the host active layer are critical in the light absorption, film morphology, and charge‐carrier mechanism of their photovoltaic blends to achieve highly efficient organic solar cells (OSCs). However, an effective third component needs to be introduced in the host binary blend as a combination of ternary blend, which can improve the absorption profile, film morphology, and charge dynamics. In this work, non‐fullerene acceptor DRCTF is incorporated as a third component in host PM6:Y6 binary blend. Compared to host blend, the ternary blend improves charge‐transfer processes, as evidenced by steady‐state photoluminescence and time‐resolved photoluminescence measurements. It is found that the addition of 20% DRCTF into host binary blend results in improved charge dissociation and transport with reduced recombination, and voltage losses. All these characteristics contribute to an improved power conversion efficiency of 14.45% in the PM6:Y6:DRCTF ternary OSCs (T‐OSCs), compared to PM6:Y6 (12.46%) in open‐air fabrication conditions. Consequently, in this study, the impact of third components on the charge‐transfer mechanism in T‐OSCs is elucidated. These findings suggested that T‐OSCs with a perfectly chosen third component in the host binary blend achieve a comprehensive absorption profile, smooth film morphology, efficient charge dynamics, and reduced voltage loss.

Quantitative Analysis of the Synergy of Doping and Nanostructuring of Oxide Photocatalysts.

In this paper, the effect of doping and nanostructuring on the electrostatic potential across the electrochemical interface between a transition metal oxide and a water electrolyte is investigated by means of the Poisson-Boltzmann model. For spherical nanoparticles and nanorods, compact expressions for the limiting potentials at which the space charge layer includes the whole semiconductor are reported. We provide a quantitative analysis of the distribution of the potential drop between the solid and the liquid and show that the relative importance changes with doping. It is usually assumed that high doping improves charge dynamics in the semiconductor but reduces the width of the space charge layer. However, nanostructuring counterbalances the latter negative effect; we show quantitatively that in highly doped nanoparticles the space charge layer can occupy a similar volume fraction as in low-doped microparticles. Moreover, as shown by some recent experiments, under conditions of high doping the electric fields in the Helmholtz layer can be as high as 100 mV/Å, comparable to electric fields inducing freezing in water. This work provides a systematic quantitative framework for understanding the effects of doping and nanostructuring on electrochemical interfaces, and suggests that it is necessary to better characterize the interface at the atomistic level.

Porphyrin-Based Hole-Transporting Materials for Perovskite Solar Cells: Boosting Performance with Smart Synthesis.

Perovskite solar cells (PSCs) are becoming a promising and revolutionary advancement within the photovoltaic field globally. Continuous improvement in efficiency, straightforward processing methods, and use of lightweight and cost-effective materials represent superior features, among other notable aspects. Still, long-term stability and durability are issues to address to facilitate widespread commercial adoption and practical application prospects. Research has focused on overcoming these challenges, and charge transport materials play a critical role in determining charge dynamics, photovoltaic performance, and device stability. Conventional hole-transporting materials (HTMs), spiro-OMeTAD and PTAA, contribute to remarkable power conversion efficiencies owing to high thin-film quality and matched energy alignment. However, they often show a high material cost, low carrier mobility, and poor stability, which greatly limit their practical applications. Now, this review outlines recent advances in synthetic approaches to porphyrin-based HTMs to tune the charge dynamics by optimizing their molecular structures and properties. The main structural features comprise porphyrins of A4-type, trans A2B2-type, and photosynthetic pigment analogues. Strategies include well-established routes to provide the required macrocycles, such as condensation of pyrrole or dipyrromethanes with suitable aldehydes, metalation of the porphyrin inner core, and postfunctionalization of peripheral positions. These functionalizations involve conventional procedures (e.g., halogenation, esterification, transesterification, nucleophilic oxidation, reduction, and nucleophilic substitution) as well as metal-catalyzed ones such as Suzuki-Miyaura, Sonogashira, Buchwald-Hartwig, and Ullmann cross-coupling reactions. As HTMs can also protect the perovskite layer from the external environment, porphyrin structures play a pivotal role in chemical, mechanical, and environmental stability, with their high hydrophobicity ability as the most significant parameter. The impact of porphyrins on the hole hopping of other HTMs while acting as an additive or an interlayer, passivating defects, and improving charge transport is also highlighted to provide real insights into ways to develop efficient and stable porphyrin-based materials for PSCs. This perspective aims to guide the scientific community in the design of new porphyrin molecules to place PSCs as an outperformer in photovoltaic technologies.

Kinetic simulations of dust grain charging in experimental plasma conditions

This paper presents fully-kinetic numerical investigations of the charging of spherical and irregular dust grains in the OML sheath regime and a stationary experimental plasma environment utilizing the Dusty Parallel Immersed-Finite-Element Particle-in-Cell (PIFE-PIC-D) framework. The simulations account for surface charging of the dust grains immersed in an stationary plasma environment. PIFE-PIC-D explicitly resolves the geometrical and material properties (permittivity) of each individual dust grain. The charge collection over time of each dust grain is investigated with varying size, irregularity, number of grains, spacing between dust grains, and permittivity. The charging behavior of a dust cluster is estimated by calculating its electron Debye length edge-to-edge separation to offer valuable insights into a dust cluster’s general charge dynamics. Lastly, unlike prior studies that focused solely on either fully conducting spheres or perfectly dielectric spheres, this work explores a more comprehensive range of permittivities for irregular dust grain aggregates.

Heavy-Metal-Free Colloidal Quantum Dots: Progress and Opportunities in Solar Technologies.

Colloidal quantum dots (QDs) hold great promise as building blocks in solar technologies owing to their remarkable photostability and adjustable properties through the rationale involving size, atomic composition of core and shell, shapes, and surface states. However, most high-performing QDs in solar conversion contain hazardous metal elements, including Cd and Pb, posing significant environmental risks. Here, a comprehensive review of heavy-metal-free colloidal QDs for solar technologies, including photovoltaic (PV) devices, solar-to-chemical fuel conversion, and luminescent solar concentrators (LSCs), is presented. Emerging synthetic strategies to optimize the optical properties by tuning the energy band structure and manipulating charge dynamics within the QDs and at the QDs/charge acceptors interfaces, are analyzed. A comparative analysis of different synthetic methods is provided, structure-property relationships in these materials are discussed, and they are correlated with the performance of solar devices. This work is concluded with an outlook on challenges and opportunities for future work, including machine learning-based design, sustainable synthesis, and new surface/interface engineering.

Fast Optimization of Charged Particle Dynamics with Damping

Fast Optimization of Charged Particle Dynamics with Damping

NiO thin films fabricated using spray-pyrolysis technique: structural and optical characterization and ultrafast charge dynamics studies

Nickel (II) oxide, NiO, is a wide band gap Mott insulator characterized by strong Coulomb repulsion between d-electrons and displays antiferromagnetic order at room temperature. NiO has gained attention in recent years as a very promising candidate for applications in a broad set of areas, including chemistry and metallurgy to spintronics and energy harvesting. Here, we report on the fabrication of polycrystalline NiO using spray-pyrolysis technique, which is a deposition technique able to produce quite uniform films of pure and crystalline materials without the need of high vacuum or inert atmospheres. The composition and structure of the NiO thin films were then studied using x-ray diffraction, and atomic force and scanning electron microscopies (SEM). The phononic and magnonic properties of the NiO thin films were also studied via Raman spectroscopy, and the ultrafast electron dynamics by using optical pump probe spectroscopy. We found that the NiO samples display the same phonon and magnon excitations expected for single crystal NiO at room temperature, and that electron dynamics in our system is like those of previously reported NiO mono- and polycrystalline systems synthesized using different techniques. These results prove that spray-pyrolysis can be used as affordable and large-scale fabrication technique to synthesize strongly correlated materials for a large set of applications.

Improvement of linearity in trap-rich substrates for radio frequency applications: Which of donor and acceptor trap is the best?

This paper investigates the effect of volume traps intentionally introduced into a silicon-on-insulator (SOI) substrate by inserting a layer of polysilicon beneath buried oxide. In radio frequency applications, this type of substrate, referred to as trap-rich, is known to considerably reduce the generation of harmonics resulting from the parasitic non-linear charge dynamics introduced by the substrate handler under the buried oxide. This analysis focuses on a test vehicle in the form of an integrated coplanar waveguide on two types of substrates, namely, high-resistivity SOI substrates with and without a trap-rich layer. From a modeling point of view, a simulation methodology is implemented in order to convert the 3D simulation of the coplanar waveguide into a 2D treatment that takes into account the wave propagation effect associated with the distributed nature of the transmission line. As a first step, this modeling strategy is implemented to reproduce the effect of increasing substrate resistivity on 2nd and 3rd harmonic reductions, leading to an excellent agreement with experimental data. Building on this validation of the simulation method, we have opted to simulate the non-linear response of the transmission line on the SOI trap-rich substrate by simplifying the trap distribution model. To avoid the adoption of unverified and strongly process-dependent trap distributions across the bandgap, a midgap monovalent trap density has been introduced, either acceptor or donor density. A monovalent density of acceptor traps with a concentration of 1016 cm−3 and a carrier lifetime of 0.1 ns has been shown to reproduce the experimental data very accurately with a substantial reduction in 2nd and 3rd harmonics. A detailed analysis of the displacement current waveforms explains the beneficial role of acceptor traps compared with donor traps.

Exploring surface charge dynamics: implications for AFM height measurements in 2D materials.

An often observed artifact in atomic force microscopy investigations of individual monolayer flakes of 2D materials is the inaccurate height derived from topography images, often attributed to capillary or electrostatic forces. Here, we show the existence of a Joule dissipative mechanism related to charge dynamics and supplementing the dissipation due to capillary forces. This particular mechanism arises from the surface conductivity and assumes significance specially in the context of 2D materials on insulating supports. In such scenarios, the oscillating tip induces in-plane charge currents that in many circumstances constitute the main dissipative contribution to amplitude reduction and, consequently, affect the measured height. To investigate this phenomenon, we conduct measurements on monolayer flakes of co-deposited graphene oxide and reduced graphene oxide. Subsequently, we introduce a general model that elucidates our observations. This approach offers valuable insights into the dynamics of surface charges and their intricate interaction with the tip.

SU(N) symmetry of coherent photons controlled by rotated waveplates

The coherent state from a laser source has spin and orbital degrees of freedom, which allow an arbitrary superposition state among orthogonal states with varying amplitudes and phases. Here, we theoretically show coherent photons with SU(N) symmetry are characterised by expectation values of angular momentum shown on a hypersphere in SO(N2−1) space. To demonstrate expected unitary transformations in experiments, we have constructed generators of transformations in the Lie group simply by combining widely available optical components such as waveplates and vortex lenses. We show a superposition state between twisted and Gaussian states is characterised by the dynamics of the topological charge upon the transformation in SU(3) states. We also realised photonic singlet and triplet states corresponding to SU(4) states, which were projected to SU(2)×SU(2) states upon passing through a rotated polariser.

Coupled Spacecraft Charging Due to Continuous Electron Beam Emission and Impact

Spacecraft charge naturally in orbit due to the plasma environment and the electromagnetic radiation from the sun. By emitting an electron beam, a servicer spacecraft can control its electric potential and also the potential of a neighboring target if the electron beam is aimed at the target spacecraft. In addition, the impacting electron beam excites secondary electrons and x-rays, providing a way to touchlessly sense the potential of the target. Because of the electron beam, the charging dynamics of the two spacecraft are coupled. This paper studies the effects of the beam on the electric potentials using a numerical charging model. It is found that multiple equilibria may exist due to the electron beam. Jumps between equilibrium configurations are possible when the electron beam energy is quickly reduced or when current fluctuations are present. Being aware of multiple equilibrium configurations is important for feedback-based charge control but also enables a new open-loop charge control around one of the equilibria. The effect of the electron beam on the spacecraft potentials is studied for geostationary Earth orbit and cislunar space. It is found that the current applied by the beam to the target may influence remote electric potential sensing methods.

Parameters estimation and sensitivity analysis of lithium-ion battery model uncertainty based on osprey optimization algorithm

To advance the field of lithium-ion battery (LIB) research, this paper unveils an accurate modelling of LIB that primarily relies on the equivalent circuit model, backed by the Osprey Optimization Algorithm (OOA). In the modelling stage, both single and double resistance-capacitance models are evaluated to depict the charge dynamics, incorporating the effects of fading, load, and temperature variations. The OOA approach is utilized to minimize integral squared errors between the actual measured and model-predicted battery voltages under constraints imposed by the model design variables. This approach is applied to a commercial 2.6 Ahr Panasonic LIB, with the performance of the OOA-based model being benchmarked against models developed by means of other optimization algorithms for further validation. Moreover, the robustness of the OOA method is assessed under battery uncertainty conditions or model parameter variation. A sensitivity analysis is performed on the battery model by employing a proposed approach that evaluates the impact of varying each parameter of the battery model by ±5 %, in a sequence that ascends and descends from 0 to 5 %. The single resistance-capacitance model is selected for in-depth validations. Notably, the OOA approach excels in estimating parameters for LIB modeling under both normal and abnormal operating conditions.

Optimal management of coupled shared autonomous electric vehicles and power grids: Potential of renewable energy integration

Shared Autonomous Electric Vehicles (SAEVs) are pivotal for future transportation, offering both promise and challenges upon integration with the power grid. This symbiosis augments power system flexibility, stability and reliability through Vehicle-to-Grid (V2G) services, and optimize transportation efficiency. However, it amplifies the demand for robust charging infrastructure and electricity power during peak periods. This paper proposes a framework employing a sequential receding horizon optimization approach to manage SAEV mobility and charging dynamics. Focused on maximizing transportation service quality while ensuring power grid stability, the model accommodates dynamic trip requests and electricity generation, utilizing a rolling horizon algorithm. Notably, the study explores the potential of SAEVs in fortifying the integration of renewable energy resources (RES) into the power grid. Our research strives to equip policymakers and system planners with a robust tool for crafting efficient and sustainable future urban transportation and energy systems.

Characteristic voltages and times from capacitance–voltage analysis of quantum dot light-emitting diodes

The characteristic voltages in the capacitance–voltage (C–V) curve of quantum dot light-emitting diodes (QLEDs) are usually linked to the start of charge injection and recombination in a working device. However, it may lead to a misunderstanding of the carrier process in QLEDs. This is because capacitance change only reflects an electrical response of additional carriers induced by a small signal loaded on an applied DC voltage but does not directly correlate with the total free carrier response governed by the working voltage. In this work, we study the frequency-dependent C–V characteristics of a blue QLED, focusing on the characteristic voltages, characteristic times, and their relationships. First of all, we identify that the charge injection point of QLEDs should be extracted by the current density–voltage–luminance characteristics rather than the C–V curve. As for the characteristic voltages obtained from the C–V curve, they are determined by voltage-dependent characteristic times in different time domains. Furthermore, the C–V characteristic is helpful to evaluate charge accumulation or leakage in blue QLED, serving as an accessible analysis tool in the carrier transport process. Our work provides a definite physical meaning of characteristic voltages in the C–V curve and exhibits the usefulness of C–V characteristics for analyzing the charge dynamics of QLED.

Indeno[3,2-b]carbazole-Based Small Molecule Layer Enables Optimized Carrier Transport for PbS Quantum Dot NIR Photodetectors.

Colloidal quantum dots (CQDs) have garnered considerable attention for photodetectors (PDs), attributable to exceptional photoelectric properties and ease solution-based processing. However, the prevalent use of 1,2-ethanedithiol (EDT) as a hole transport layer (HTL) has limitations, such as energy level discrepancies, requisite oxidation, and intricate multilayer assembly. Organic p-type materials, lauded for their superior attributes and synthetic versatility, are now stepping forward as viable substitutes for conventional EDT HTLs. In this work, we introduced an organic HTL derived from indolo[3,2-b]carbazole, named ZL004, leading to a marked improvement in carrier generation and collection, facilitated by the optimized band alignment and enhanced interfacial charge dynamics. The ZL004-based PDs exhibit a photoresponsivity of 0.45 A/W, a noise current of 1.8 × 10-11 A Hz-0.5, a specific detectivity of 4.6 × 109 Jones, and an expansive linear dynamic range of 107 dB─surpassing EDT-based devices across the board, demonstrating the extraordinary property of organic p-type materials for CQD-based PDs.

Exciton-Phonon Coupling Induces a New Pathway for Ultrafast Intralayer-to-Interlayer Exciton Transition and Interlayer Charge Transfer in WS2-MoS2 Heterostructure: A First-Principles Study.

Despite the weak, van der Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50 fs time scales. An early theoretical understanding of charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate optical properties of such materials. We employ an ab initio many-body perturbation theory approach, which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS2/MoS2 heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photoexcited excitons at the K valley of MoS2 and WS2 is 67 and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways that are otherwise inaccessible to noninteracting electrons and holes.

Innovative lignin photocatalyst system motivated by intramolecular charge dynamics for H2O2 production

Lignin is the most abundant renewable resource with aromatic ring structures on earth, however, its intrinsic redox functional region for H2O2 production has not been fully identified due to the source diversity and structure complexity. Herein, the charge dynamics between the HOMO level with electron-donating capability and the LOMO level with electron-accepting capability are comprehensively clarified by using 14 types of customized lignin models as photocatalytic systems. The electron-transport patterns are modulated by the structure–activity relationship of effective oxygen-containing functional groups, including conjugation effects of methoxy groups, induced effects of hydroxyl or carbonyl groups, and ether bonds determining the overlap ratio of energy levels. A comprehensive evaluation and screening system is established to identify efficient lignin photocatalysts extracted from various sources and processes that are suitable for specific application scenarios, combined with theoretical calculations and molecular dynamics simulations. The fledgling H2O2 production efficiency is expected to be further improved through constructive guidance on interfering factors. The report on direct H2O2 production by O2 reduction and the action mechanism of the novel lignin photocatalyst will draw a new blueprint for the breakthrough of bio-photocatalytic functionalization.

Electronic structure orientation as a map of in-plane antiferroelectricity in β'-In2Se3.

Antiferroelectric (AFE) materials are excellent candidates for sensors, capacitors, and data storage due to their electrical switchability and high-energy storage capacity. However, imaging the nanoscale landscape of AFE domains is notoriously inaccessible, which has hindered development and intentional tuning of AFE materials. Here, we demonstrate that polarization-dependent photoemission electron microscopy can resolve the arrangement and orientation of in-plane AFE domains on the nanoscale, despite the absence of a net lattice polarization. Through direct determination of electronic transition orientations and analysis of domain boundary constraints, we establish that antiferroelectricity in β'-In2Se3 is a robust property from the scale of tens of nanometers to tens of micrometers. Ultimately, the method for imaging AFE domain organization presented here opens the door to investigations of the influence of domain formation and orientation on charge transport and dynamics.