Charge Dynamics Research Articles

Unveiling the Crucial Impact of Defects in the Hole Transport Layer on the Efficiency of Phosphorescent Organic Light‐Emitting Diodes

AbstractThe ongoing challenge in advancing organic light‐emitting diodes (OLEDs) is to mitigate performance degradation during operation, caused by the dissociation of molecular components and the ensuing defect formation. While defects can emerge throughout OLEDs with diverse detrimental effects, prior research has predominantly focused on those within the emission layer (EML), leaving other device areas unexplored. In this work, a previously overlooked, yet profound impact of defects in the hole transport layer (HTL) on charge and exciton dynamics in a phosphorescent OLED (PHOLED) is investigated. HTL defects confine and impede the injection of free holes into the EML, requiring an increased supply of electrons to maintain constant current in the device. Consequently, excitons become concentrated at the HTL/EML interface, leading to enhanced bimolecular annihilations and an 8% reduction in device efficiency. Furthermore, when HTL defects are within 8 nm from the interface, they can directly quench excitons, resulting in a substantial decrease in device efficiency by up to 76%. This defect‐induced exciton quenching occurs through both Dexter and Förster energy transfer, each with distinct distance dependence. The findings emphasize the necessity of preventing defect formation, particularly in proximity to the HTL/EML interface, to minimize efficiency degradation for PHOLEDs.

Rejoint of Carbon Nitride Fragments into Multi-Interfacial Order-Disorder Homojunction for Robust Photo-Driven Generation of H2 O2.

Photocatalytic technology based on carbon nitride (C3 N4 ) offers a sustainable and clean approach for hydrogen peroxide (H2 O2 ) production, but the yield is severely limited by the sluggish hot carriers due to the weak internal electric field. In this study, a novel approach is devised by fragmenting bulk C3 N4 into smaller pieces (CN-NH4 ) and then subjecting it to a directed healing process to create multiple order-disorder interfaces (CN-NH4 -NaK).The resulting junctions in CN-NH4 -NaK significantly boost charge dynamics and facilitate more spatially and orderly separated redox centers. As a result, CN-NH4 -NaK demonstrates outstanding photosynthesis of H2 O2 via both two-step single-electron and one-step double-electron oxygen reduction pathways, achieving a remarkable yield of 16675µmol h-1 g-1 , excellent selectivity (>91%), and a prominent solar-to-chemical conversion efficiency exceeding 2.3%. These remarkable results surpass pristine C3 N4 by 158 times and outperform previously reported C3 N4 -based photocatalysts. This work represents a significant advancement in catalyst design and modification technology, inspiring the development of more efficient metal-free photocatalysts for the synthesis of highly valued fuels.

Asymmetric Π-Bridge Engineering Enables High-Permittivity Benzo[1,2-B:4,5-b']Difuran-Conjugated Polymer for Efficient Organic Solar Cells.

Organic solar cells (OSCs) exhibit complex charge dynamics, which are closely correlated with the dielectric constant (ɛr ) of photovoltaic materials. In this work, a series of novel conjugated copolymers based on benzo[1,2-b:4,5-b']difuran (BDF) and benzotriazole (BTz) is designed and synthesized, which differ by the nature of π-bridge from one another. The PBDF-TF-BTz with asymmetric furan and thiophene π-bridge demonstrates a larger ɛr of 4.22 than PBDF-dT-BTz with symmetric thiophene π-bridge (3.15) and PBDF-dF-BTz with symmetric furan π-bridge (3.90). The PBDF-TF-BTz also offers more favorable molecular packing and appropriate miscibility with non-fullerene acceptor Y6 than its counterparts. The corresponding PBDF-TF-BTz:Y6 OSCs display efficient exciton dissociation, fast charge transport and collection, and reduced charge recombination, eventually leading to a power conversion efficiency of 17.01%. When introducing a fullerene derivative (PCBO-12) as a third component, the PBDF-TF-BTz:Y6:PCBO-12 OSCs yield a remarkable FF of 80.11% with a high efficiency of 18.10%, the highest value among all reported BDF-polymer-based OSCs. This work provides an effective approach to developing high-permittivity photovoltaic materials, showcasing PBDF-TF-BTz as a promising polymer donor for constructing high-performance OSCs.

Real-time dynamics of axial charge and chiral magnetic current in a non-Abelian expanding plasma

Real-time dynamics of axial charge and chiral magnetic current in a non-Abelian expanding plasma

Hybrid random batch idea and nonlinear conjugate gradient method for accelerating charged polymer dynamics simulation

Hybrid random batch idea and nonlinear conjugate gradient method for accelerating charged polymer dynamics simulation

Mapping Light-Dressed Floquet Bands by Highly Nonlinear Optical Excitations and Valley Polarization.

Ultrafast nonlinear optical phenomena in solids have been attracting a great deal of interest as novel methodologies for the femtosecond spectroscopy of electron dynamics and control of the properties of materials. Here, we theoretically investigate strong-field nonlinear optical transitions in a prototypical two-dimensional material, hBN, and show that the k-resolved conduction band charge occupation patterns induced by an elliptically polarized laser can be understood in a multiphoton resonant picture, but, remarkably, only if using the Floquet light-dressed states instead of the undressed matter states. Our work demonstrates that Floquet dressing affects ultrafast charge dynamics and photoexcitation even from a single pump pulse and establishes a direct measurable signature for band dressing in nonlinear optical processes in solids, opening new paths for ultrafast spectroscopy and valley manipulation.

Molecular Understanding of Charging Dynamics in Supercapacitors with Porous Electrodes and Ionic Liquids.

Porous electrodes and ionic liquids could significantly enhance the energy storage of supercapacitors. However, they may reduce the charging dynamics and power density due to the nanoconfinement of porous electrodes and the high viscosity of ionic liquids. A comprehensive understanding of the charging mechanism in porous supercapacitors with ionic liquids provides a crucial theoretical foundation for their design optimization. Here, we review the progress of molecular simulations of the charging dynamics in supercapacitors consisting of porous electrodes and ionic liquids. We highlight and delve into the breakthroughs in the ion transport and charging mechanism for electrodes with subnanometer pores and realistic porous structures. We also discuss future directions for the charging dynamics of supercapacitors.

Dynamics of unipolar charged particles flowing through a cylindrical tube at high number concentrations

This work provides a theoretical and non-dimensional numerical analysis of the dynamics of gas-borne unipolar charged particles flowing through a cylindrical tube at high number concentrations. It is discovered that above a threshold input particle number concentration, the concentration at the exit of the tube is nearly constant and independent of further increase in the input. Based on a maximum of 2% sensitivity of the system, the threshold input number concentration is quantified for a tube of any length more than 40 × the radius. The theoretical analysis developed in this work is used to derive correlations for the threshold input concentration and the exit concentration, as well as show that the exit particle concentration is lower than the threshold input value by a factor of 20/PD, where PD is the well-known diffusive particle penetration. The analysis demonstrates that a constant concentration can occur if and only if the ratio of non-dimensional electric particle mobility to the Gormley-Kennedy diffusion parameter is above 74, which is a hitherto unexplored regime of charged aerosol flow dynamics. It is shown that the constant concentration can occur in compact tube designs (L/R<100) as long as the mass Peclet number of the particles is high and reducing the mass Peclet number to around 10 or below makes it impossible to obtain an input-independent number concentration. It is also shown that the Reynolds number of the flow has very little effect on the charged transport dynamics, where the order of magnitude of the input-independent number concentration is not highly sensitive to the Reynolds number, provided the system remains laminar. Finally, practical examples for designing instrumentation to generate a constant concentration of charged particles are discussed.

Time calibration studies for the Timepix3 hybrid pixel detector in electron microscopy

Direct electron detection is currently revolutionizing many fields of electron microscopy due to its lower noise, its reduced point-spread function, and its increased quantum efficiency. More specifically to this work, Timepix3 is a hybrid-pixel direct electron detector capable of outputting temporal information of individual hits in its pixel array. Its architecture results in a data-driven detector, also called event-based, in which individual hits trigger the data off the chip for readout as fast as possible. The presence of a pixel threshold value results in an almost readout-noise-free detector while also defining the hit time of arrival and the time the signal stays over the pixel threshold. In this work, we have performed various experiments to calibrate and correct the Timepix3 temporal information, specifically in the context of electron microscopy. These include the energy calibration, and the time-walk and pixel delay corrections, reaching an average temporal resolution throughout the entire pixel matrix of 1.37±0.04ns. Additionally, we have also studied cosmic rays tracks to characterize the charge dynamics along the volume of the sensor layer, allowing us to estimate the limits of the detector’s temporal response depending on different bias voltages, sensor thickness, and the electron beam ionization volume. We have estimated the uncertainty due to the ionization volume ranging from about 0.8 ns for 60 keV electrons to 8.8 ns for 300 keV electrons.

Implementation of a Robust PMAE Algorithm to Limit Large Gain Variation in a Fuel-Cell Integrated EV Charging Architecture

This paper proposes an adaptive error convergence technique to improve the charging dynamics of an electric vehicle (EV) charging system in a grid-integrated proton-exchange membrane fuel cell (PEMFC) architecture. The fuel cell is most often characterized by its slow dynamics during multiple load transients and preferably operated as an auxiliary energy source in EV charging architecture. The delayed response affects the overall charging dynamics with further controller phase lag and non-linear system parameters to induce a significant convergence error during reference command tracking. To effectively control multi-source integrated charging dynamics in an EV charging architecture, a projection-modified adaptive estimation (PMAE) algorithm is proposed in this paper. The control algorithm establishes a variable gain adaptation within a bounded error contour and updates the controller gains based on the tracking error dynamics gradient. The fuel cell, supporting other energy resources, provides a stronger charging resiliency with a duty-modified control technique at the EV charging station. To validate the effectiveness of the controller implementation, a grid-integrated multi-objective EV charging prototype is designed and developed for grid-to-vehicle (G2V) and vehicle-to-grid (V2G) at variable operating conditions.

Effective photocatalytic hydrogen evolution by Ti3C2-modified CdS synergized with N-doped C-coated Cu2O in S-scheme heterojunctions

Photocatalytic hydrogen evolution through water splitting holds tremendous promise for converting solar energy into a clean and renewable fuel source. However, the efficiency of photocatalysis is often hindered by poor light absorption, insufficient charge separation, and slow reaction kinetics of the photocatalysts. In this study, we designed and synthesized a novel S-scheme heterojunction comprising Ti3C2 MXene, CdS nanorods, and nitrogen-doped carbon coated Cu2O (Cu2O@NC) core-shell nanoparticles. Ti3C2 MXene as a cocatalyst enhances the light absorption and charge transfer of CdS nanorods. Simultaneously, the core-shell Cu2O@NC nanoparticles establish a pathway for transferring photogenerated electrons and create a favorable band alignment for efficient hydrogen evolution. The synergistic effects of Ti3C2 MXene and Cu2O@NC on CdS nanorods result in multiple charge transfer channels and improved photocatalytic performance. The optimal hydrogen evolution rate of the Ti3C2-CdS-Cu2O@NC S-scheme heterojunction photocatalyst is 7.4 times higher than that of pure CdS. Experimental techniques and DFT calculations were employed to explore the structure, morphology, optical properties, charge dynamics, and band structure of the heterojunction. The results revealed that the S-scheme mechanism effectively suppresses the recombination of photogenerated carriers and facilitates the separation and migration of photogenerated electrons and holes to the reaction sites. Furthermore, Ti3C2 MXene provides abundant active sites essential for accelerating the surface H2-evolution reaction kinetics. The Cu2O@NC core-shell nanoparticles with a large surface area and high stability are closely adhered to CdS nanorods and establish an S-scheme internal electric field with CdS nanorods to drive charge separation. This investigation provides valuable insights into the rational design of CdS-based photocatalysts, enabling efficient hydrogen production by harnessing the robust kinetic driving force provided by the S-scheme heterojunctions.

Simulation of Charge Dynamics of a Photocathode Semiconductor Layer

Simulation of Charge Dynamics of a Photocathode Semiconductor Layer

Efficient ANFIS-Driven Power Extraction and Control Strategies for PV-BESS Integrated Electric Vehicle Charging Station

Amid escalating concerns over global warming and climate shifts, the imperative for advanced transportation solutions intensifies, aiming to balance economic growth and environmental preservation. The worldwide upsurge in electric vehicle (EV) adoption reflects this transformative aspiration, yet relying solely on fossil fuel-based EV charging infrastructure falls short of addressing climate challenges. In response, Multi Energy Integrated EV Charging Stations have emerged as a pragmatic solution, seamlessly blending renewable energy sources, grid power, and EV charging needs. This study proposes a meticulously crafted charging station design employing Adaptive Neuro-Fuzzy Inference System (ANFIS) technology for voltage-controlled Maximum Power Point Tracking (MPPT), coupled with Type 3 & 2 controllers, which collaborate in harmonious precision to streamline EV charging dynamics and Neural Network-based grid management. The design is meticulously assessed and evaluated using MATLAB/Simulink, presenting a promising avenue for sustainable and efficient EV charging infrastructure in the face of evolving energy demands and environmental concerns.

Visualization of Charge Dynamics when Water Droplets Bounce on a Hydrophobic Surface.

Visualizing the motion of water droplets and understanding their electrification behavior holds significance for applications related to droplet transport, self-cleaning, and anti-icing/deicing and for providing a comprehensive explanation of the solid-liquid triboelectrification mechanism. Here, by constructing microcolumnar structures on the polytetrafluoroethylene surface, a water droplet-based single electrode triboelectric nanogenerator was fabricated for visualizing charge dynamics when a water droplet bounces on a hydrophobic surface. The motion state of the water droplet is closely linked to its electrification behavior through the integration of a high-speed camera and an ammeter. The electrification behavior stemming from the bounce of the water droplet is dynamically captured in real-time. The results show that the magnitude and polarity of the electrical signal have strong dependence on the motion state of the water droplet. For instance, when a water droplet approaches or moves away from the substrate in a single direction, a unipolar electrical signal is generated. However, when the water droplet reaches its limit in the initial motion direction, it signifies a static equilibrium state, resulting in the electrical signal being at zero. Furthermore, we examine the impact of factors such as impact speed, drop contact area, contact line spreading/retraction speed, and impact angle on electrification. Finally, based on the close relationship between poly(ethylene oxide) (PEO) droplet bounce dynamics and electrical signals, the bouncing details of PEO droplets with different concentrations are tracked by electrical signals. This study digitally presents the whole process of droplet bounce in situ and provides a means for monitoring and tracking droplet movement.

Anomalous efficiency elevation of quantum-dot light-emitting diodes induced by operational degradation

Quantum-dot light-emitting diodes promise a new generation of high-performance and solution-processed electroluminescent light sources. Understanding the operational degradation mechanisms of quantum-dot light-emitting diodes is crucial for their practical applications. Here, we show that quantum-dot light-emitting diodes may exhibit an anomalous degradation pattern characterized by a continuous increase in electroluminescent efficiency upon electrical stressing, which deviates from the typical decrease in electroluminescent efficiency observed in other light-emitting diodes. Various in-situ/operando characterizations were performed to investigate the evolutions of charge dynamics during the efficiency elevation, and the alterations in electric potential landscapes in the active devices. Furthermore, we carried out selective peel-off-and-rebuild experiments and depth-profiling analyses to pinpoint the critical degradation site and reveal the underlying microscopic mechanism. The results indicate that the operation-induced efficiency increase results from the degradation of electron-injection capability at the electron-transport layer/cathode interface, which in turn leads to gradually improved charge balance. Our work provides new insights into the degradation of red quantum-dot light-emitting diodes and has far-reaching implications for the design of charge-injection interfaces in solution-processed light-emitting diodes.

Data-Driven Discovery of Lithium-Ion Battery State of Charge Dynamics

Abstract We present a physics-inspired input/output predictor of lithium-ion batteries (LiBs) for online state-of-charge (SOC) prediction. The complex electrochemical behavior of batteries results in nonlinear and high-dimensional dynamics. Accurate SOC prediction is paramount for increased performance, improved operational safety, and extended longevity of LiBs. The battery's internal parameters are cell-dependent and change with operating conditions and battery health variations. We present a data-driven solution to discover governing equations pertaining to SOC dynamics from battery operando measurements. Our approach relaxes the need for detailed knowledge of the battery's composition while maintaining prediction fidelity. The predictor consists of a library of candidate terms and a set of coefficients found via a sparsity-promoting algorithm. The library was enhanced with explicit physics-inspired terms to improve the predictor's interpretability and generalizability. Further, we developed a Monte Carlo search of additional nonlinear terms to efficiently explore the high-dimensional search space and improve the characterization of highly nonlinear behaviors. Also, we developed a hyperparameter autotuning approach for identifying optimal coefficients that balance accuracy and complexity. The resulting SOC predictor achieved high predictive performance scores (RMSE) of 2.2×10−6 and 4.8×10−4, respectively, for training and validation on experimental results corresponding to a stochastic drive cycle. Furthermore, the predictor achieved an RMSE of 8.5×10−4 on unseen battery measurements corresponding to the standard US06 drive cycle, further showcasing the adaptability of the predictor and the enhanced modeling approach to new conditions.

Watching the Interplay between Photoinduced Ultrafast Charge Dynamics and Nuclear Vibrations.

Here is presented the ultrafast hole-electron dynamics of photoinduced metal to ligand charge-transfer (MLCT) states in a Ru(II) complex, [Ru(dcbpy)2(NCS)2]4- (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), a photoactive molecule employed in dye sensitized solar cells. Via cutting-edge computational techniques, a tailored computational protocol is here presented and developed to provide a detailed analysis of the electronic manifold coupled with nuclear vibrations to better understand the nonradiative pathways and the resulting overall dye performances in light-harvesting processes (electron injection). Thus, the effects of different vibrational modes were investigated on both the electronic levels and charge transfer dynamics through a theoretical-computational approach. First, the linear response time-dependent density functional (LR-TDDFT) formalism was employed to characterize excitation energies and spacing among electronic levels (the electronic layouts). Then, to understand the ultrafast (femtosecond) charge dynamics on the molecular scale, we relied on the nonperturbative mean-field quantum electronic dynamics via real-time (RT-) TDDFT. Three vibrational modes were selected, representative for collective nuclear movements that can have a significant influence on the electronic structure: two involving NCS- ligands and one involving dcbpy ligands. As main results, we observed that such MLCT states, under vibrational distortions, are strongly affected and a faster interligand electron transfer mechanism is observed along with an increasing MLCT character of the adiabatic electronic states approaching closer in energy due to the vibrations. Such findings can help both in providing a molecular picture of multidimensional vibro-electronic spectroscopic techniques, used to characterize ultrafast coherent and noncoherent dynamics of complex systems, and to improve dye performances with particular attention to the study of energy or charge transport processes and vibronic couplings.

Investigation on AC and DC Breakdown Mechanism of Surface-Ozone-Treated LDPE Films under Varied Thicknesses.

Electrical breakdown is an important physical phenomenon in power equipment and electronic devices. Recently, the mechanism of AC and DC breakdown has been preliminarily revealed as electrode-dielectric interface breakdown and bulk breakdown, respectively, based on space charge dynamics through numerical calculations. However, the AC breakdown mechanism still lacks enough direct experimental support, which restricts further understanding and the design and development of electrical structures. Here, in this study, LDPE films with various thicknesses ranging from 33 μm to 230 μm were surface modified with ozone for different durations to experimentally investigate DC and AC breakdown mechanism. The results indicate that carbonyl groups (C=O) were introduced onto the film surface, forming shallow surface traps and leading to a decreased average trap depth and an increased trap density. Such a surface oxidation modulated trap distribution led to enhanced space charge injection and bulk electrical field distortion, which decreased DC breakdown strength as the oxidation duration went longer, in all film thicknesses. However, such decreases in breakdown strength occurred only in films below 55 μm under AC stresses, as the enhanced electrical field distortion at the electrode-dielectric interface was more obvious and dominating in thin films. These experimental results further confirm the proposed electrode-dielectric interface breakdown of dielectric films and provide new understandings of space charge modulated electrical breakdown, which fulfills dielectric breakdown theory and benefits the miniaturization of power equipment and electronic devices.

On the influence of humidity on the breakdown strength of air—with a case study on the PDIV of contacting enameled wire pairs

The partial discharge inception voltage (PDIV) in contacting enameled wire pairs exhibits a marked decrease with increased air humidity. While existing literature mentions several potential mechanisms for this reduction, a comprehensive quantitative assessment of the associated effects is lacking. This research paper addresses this knowledge gap by providing a quantitative estimation of the combined impact of water on the gas’s ionization yield (effective ionization coefficient) and the modification of the gap electric field caused by water absorption into the bulk of the insulating coating and the associated microscopic and macroscopic polarization processes (dielectric permittivity). However, a comparison of the theoretical predictions with experimental data reveals that these factors alone cannot fully account for the observed reduction in PDIV. Therefore, the study explores additional mechanisms mentioned in the literature, with particular focus on the development of a semi-conductive layer on the insulation coating in humid atmospheres. The numerical simulations of the surface charge dynamics within this layer suggests that the frequency-dependent decrease in PDIV under high-humidity atmospheres can indeed be attributed to the modification of the gap electric field due to the accumulation of surface charge in the semi-conductive layer.

Ultrathin nanolayer constituted by a natural polysaccharide achieves “egg-box” structured SnO2 nanoparticles toward efficient and stable perovskite solar cells

Notorious nonradiative recombination and retarded charge dynamics corrodes the performance of perovskite solar cells (PSCs). Accordingly, a natural polysaccharide sodium alginate (SA) is introduced into SnO2 colloid solution, developing “egg-box” structured SnO2 nanoparticles wrapped by ultrathin SA nanolayer to achieve a bottom-up holistic passivation strategy. The active oxygen-containing functional groups with lone pair electrons of SA can coordinate with uncoordinated Sn4+/Pb2+. Meanwhile, hydroxyl groups can form hydrogen bonds with I of perovskite. Additionally, Na+ can fill the cation vacancies in perovskite and electrostatically interacts with the halogen ions, hindering the formation of iodine Frenkel defects. Based on the synergistic effects of various functional groups, SA-modification can simultaneously inhibit the agglomeration of SnO2 nanoparticles, improve carrier dynamics, release tensile stress and promote crystallization of perovskite. Finally, the SA-optimized methylammonium-free (MA-free) device fabiricated entirely in ambient air achieves a champion efficiency up to 23.61%. The optimized devices without encapsulation exhibit better thermal and moisture stability, with the PCE retaining 80.2% of its initial efficiency after 1080 h of thermal aging at 60 °C and 91.1% of its initial efficiency after 1440 h at 25% relative humidity, respectively.