Accumulation Of Charge Carriers Research Articles

Interior Exciton Extraction by Spatial-Controlled Iodine Doping in BiOBr Photocatalysts.

Extracting interior photoinduced species to the surface before their recombination is of great importance in pursuing high-efficiency semiconductor-based photocatalysis. Traditional strategies toward charge-carrier extraction, mostly relying on the construction of an electric field gradient, would be invalid toward the neutral-exciton counterpart in low-dimensional systems. In this work, by taking bismuth oxybromide (BiOBr) as an example, we manipulate interior exciton extraction to the surface by implementing iodine doping at the edges of BiOBr plates. Spatial- and time-resolved spectroscopic analyses verified the accumulation of excitons and charge carriers at the edges of iodine-doped BiOBr (BiOBr-I) plates. This phenomenon could be associated with interior exciton extraction, driven by an energy-level gradient between interior and edge exciton states, and the following exciton dissociation processes. As such, BiOBr-I shows remarkable performance in photocatalytic C-H fluorination, mediated by both energy- and charge-transfer processes. This work uncovers the importance of spatial regulation of excitonic properties in low-dimensional semiconductor-based photocatalysis.

Simultaneous Band Alignment Modulation and Carrier Dynamics Optimization Enable Highest Efficiency in Cd‐Free Sb2Se3 Solar Cells

AbstractAntimony selenide (Sb2Se3) has developed as an eco‐friendly photovoltaic candidate owing to its non‐toxic composition and exceptional optoelectronic properties. However, the toxic and parasitic light‐absorbing CdS are widely used as electron transport layer (ETL) in Sb2Se3 solar cells, which severely limits its development. Herein, an alternative, zinc tin oxide (ZTO) ETL with varying composition‐dependent energy structure is deposited by atomic layer deposition (ALD) technique and used for constructing Cd‐free Sb2Se3 solar cells. It has been found that the ZTO ETL possessing an appropriate Zn/Sn ratio can alter the Sb2Se3/ZTO heterojunction band alignment to an ideal “spike‐like” arrangement. It not only suppresses the accumulation and recombination of charge carriers at the interface, but also effectively enhances carrier transport. In addition, thanks to the formation of passivated Sb2O3 ultra‐thin layer upon ALD process, the non‐radiative recombination within bulk Sb2Se3 can be effectively suppressed, and therefore enhancing carrier lifetime, extraction efficiency, and collection efficiency. Consequently, the as‐fabricated Mo/Sb2Se3/ZTO/ITO/Ag thin‐film solar cell demonstrates an impressive efficiency of 8.63%. This accomplishment establishes it as the most efficient Cd‐free Sb2Se3 solar cell to date, underscoring the significant advantages of incorporating ZTO ETL in the development of Sb2Se3 photovoltaic scenarios.

Exploring the influence of the contact resistance on perovskite phototransistors

Organic–inorganic hybrid perovskites are widely used in photodetection owing to their high optical absorption coefficients. A variety of research has been conducted on perovskite phototransistors and their optoelectronic properties, but the exploration of the influence of contact resistance remains limited. In this work, we employed the transmission-line method to separate the contact resistance Rc × W (ranging from 4.81 × 104 to 4.77 × 103 Ω cm) and the channel resistance Rch × W (ranging from 1.93 × 104 to 1.16 × 104 Ω cm) of (PEA)2SnI4 perovskite phototransistors at different light intensities (520 nm, ranging from 0 to 2550 μW/cm2). Further analysis reveals that illumination-induced accumulation of charge carriers at the metal/semiconductor interface reduces the Schottky barrier. Approximately 90% of the observed increase in photocurrent can be attributed to the reduction in the contact resistance. Our finding underscores the crucial role of charge injection in influencing perovskite-based phototransistors.

Modeling of Photovoltaic Characteristics of a TiO<sub>2</sub>/Porous-Si/Si-Based Heterojunction Solar Cell

This work is focused on the study of photosensitive structures based on porous Si and film TiO2, which are promising for solar energy. For numerical simulation of the transportation and accumulation of charge carriers in the considered heterostructure, the drift-diffusion approximation of the semiclassical approach was proposed. A device scheme of a solar cell model based on TiO2/porous-Si/Si heterostructures is proposed. Production of photoconverters of solar cells based on the TiO2/porous-Si/Si heterostructure can be carried out according to the standard method supplemented by additional technological operations. Ohmic contacts are formed in the upper and lower parts of the structure above the TiO2 and Si layers. The strip system of contacts is a contact grid, with hatching, the surface coefficient should not exceed 5%. The thickness of the applied layer of photoresist should be 1 μm. Using the PC1D program, the light characteristics of the fabricated structure were calculated (open circuit voltage VOC, short circuit current ISC, fill factor FF and efficiency η), and current-voltage characteristics were plotted. The influence of the thickness and doping level Nd and Na of porous Si and TiO2 layers on the productivity of a heterojunction solar cell TiO2/porous-Si/Si was studied in order to obtain a device with a good conversion efficiency. It was found that the energy conversion efficiency of a TiO2/porous-Si/Si solar cell can reach 22.5 %. Based on the optimized simulation conditions, it was found that the maximum solar cell efficiency is achieved at thicknesses of 100 and 200 nm and donor concentration of Nd=1∙1017 cm-3 and acceptor concentration of Na=1∙1018 cm-3 for TiO2 and porous Si buffer layer, accordingly.

Regulating Charge Transport Dynamics at the Buried Interface and Bulk of Perovskites by Tailored‐phase Two‐dimensional Crystal Seed Layer

AbstractTargeting the trap‐assisted non‐radiative recombination losses and photochemical degradation occurring at the interface and bulk of perovskite, especially the overlooked buried bottom interface, a strategy of tailored‐phase two‐dimensional (TP‐2D) crystal seed layer has been developed to improve the charge transport dynamics at the buried interface and bulk of perovskite films. Using this approach, TP‐2D layer constructed by TP‐2D crystal seeds at the buried interface can induce the formation of homogeneous interface electric field, which effectively suppress the accumulation of charge carriers at the buried interface. Additionally, the presence of TP‐2D crystal seed has a positive effect on the crystallization process of the upper perovskite film, leading to optimized crystal quality and thus promoted charge transport inside bulk perovskites. Ultimately, the best performing PSCs based on TP‐2D layer deliver a power conversion efficiency of 24.58 %. The devices exhibit an improved photostability with 88.4 % of their initial PCEs being retained after aging under continuous 0.8‐sun illumination for 2000 h in air. Our findings reveal how to regulate the charge transport dynamics of perovskite bulk and interface by introducing homogeneous components.

Regulating Charge Transport Dynamics at the Buried Interface and Bulk of Perovskites by Tailored-phase Two-dimensional Crystal Seed Layer.

Targeting the trap-assisted non-radiative recombination losses and photochemical degradation occurring at the interface and bulk of perovskite, especially the overlooked buried bottom interface, a strategy of tailored-phase two-dimensional (TP-2D) crystal seed layer has been developed to improve the charge transport dynamics at the buried interface and bulk of perovskite films. Using this approach, TP-2D layer constructed by TP-2D crystal seeds at the buried interface can induce the formation of homogeneous interface electric field, which effectively suppress the accumulation of charge carriers at the buried interface. Additionally, the presence of TP-2D crystal seed has a positive effect on the crystallization process of the upper perovskite film, leading to optimized crystal quality and thus promoted charge transport inside bulk perovskites. Ultimately, the best performing PSCs based on TP-2D layer deliver a power conversion efficiency of 24.58 %. The devices exhibit an improved photostability with 88.4 % of their initial PCEs being retained after aging under continuous 0.8-sun illumination for 2000 h in air. Our findings reveal how to regulate the charge transport dynamics of perovskite bulk and interface by introducing homogeneous components.

Space–Charge‐Limited Photocurrent as a Possible Cause for Low Power Conversion Efficiency in GaInN/GaN‐Based Optoelectronic Semiconductors

This study attempts to understand the cause of low power conversion efficiency (PCE) in III‐nitride optoelectronic semiconductors under optical operation. For this purpose, a GaInN/GaN heterojunction semiconductor is fabricated, and the photoexcited current–voltage (PEJV) curves are carefully measured depending on the optical excitation power and temperature. The results show unexpected excitation power‐ and temperature‐dependent behaviors, that is, the PCE decreases with increasing excitation power and increases with increasing temperature. To understand this, the space–charge‐limited photocurrent (JPh,SCL) theory (also referred to as Goodman and Rose theory) is employed, where the accumulated charge carriers in the active layer play a significant role. The conduction of JPh,SCL is ascertained by analyzing the PEJV curves. The conduction of JPh,SCL is investigated as a possible cause of the low PCE, revealing that the conduction of JPh,SCL could limit the high‐power operation of the device.

Highly efficient emerging Ag2BaTiSe4 solar cells using a new class of alkaline earth metal-based chalcogenide buffers alternative to CdS

Cu2ZnSn(S,Se)4 is a non-toxic, earth-abundant photovoltaic absorber. However, its efficiency is limited by a large open circuit voltage (VOC) deficit occurring due to its antisite defects and improper band alignment with toxic CdS buffer. Therefore, finding an absorber and non-toxic buffers that reduce VOC deficit is crucial. Herein, for the first time, Ag2BaTiSe4 is proposed as an alternative absorber using SCAPS-1D wherein a new class of alkaline earth metal chalcogenide such as MgS, CaS, SrS, and BaS is applied as buffers, and their characteristics are compared with CdS to identify their potential and suitability. The buffer and absorber properties are elucidated by tuning their thickness, carrier concentration, and defect density. Interestingly, optimization of the buffer’s carrier concentration suppressed the barrier height and accumulation of charge carriers at the absorber/buffer interface, leading to efficiencies of 18.81%, 17.17%, 20.6%, 20.85%, 20.08% in MgS, CaS, SrS, BaS, and CdS-based solar cells respectively. Upon optimizing Ag2BaTiSe4, MoSe2, and interface defects maximum efficiency of > 28% is achieved with less VOC loss (~ 0.3 V) in all solar cells at absorber’s thickness, carrier concentration, and defect density of 1 µm, 1018 cm−3, 1015 cm−3 respectively, underscoring the promising nature of Ag2BaTiSe4 absorber and new alkaline earth metal chalcogenide buffers in photovoltaics.

Probing the Recombination Process at the Intermixed Interface of a Solution-Processed Organic Light Emitting Diode By Impedance Spectroscopy

Solution-processed organic light emitting diode (s-OLED) has garnered considerable research interest due to its cost-effectiveness, improved energy efficiency, and material-saving advantages over conventional vacuum deposition methods in OLED production. Recent studies have shown that the thermal annealing process of multilayered s-OLEDs induces intermixing at the interfaces of the layers and enhances the device performance [1,2]. However, the physical origin of the enhanced s-OLED performance by intermixing at the interfaces is poorly understood. To address this issue, we utilized electrical impedance spectroscopy (IS) to investigate the thermal-induced modifications of charge carrier dynamics in a bilayer s-OLED comprising an electron transporting layer (ETL) and an emission layer (EML). We separately assessed the conductance of the ETL and the charge carrier accumulation at the EML/ETL interface by the characteristic relaxation frequency and the plateau at a low frequency regime (20~100 Hz) extracted from the frequency-dependent capacitance spectra, respectively. The IS analysis revealed that improved charge transport in the ETL by thermal annealing process was mainly responsible for the performance enhancement, rather than intermixing at the EML/ETL interface. Notably, we found that annealing the s-OLEDs above the glass transition temperature (Tg) of the ETL, at which significant intermixing occurs, increased the magnitude of negative capacitance, indicating the increased rate of nonradiative trap-assisted recombination [3]. Accordingly, our IS analysis clarified that extended intermixing at the EML/ETL interface is rather detrimental to the performance of s-OLEDs. This study demonstrates that IS can serve as a powerful tool for accurately analyzing charge carrier dynamics in s-OLEDs by simultaneously probing charge transport, interfacial charge accumulation, and recombination processes, thereby providing valuable insights for optimizing s-OLED performance.[1] ACS Appl. Mater. Interfaces 2015, 7 (37), 20779–20785. https://doi.org/10.1021/acsami.5b05818.[2] Advanced Materials Interfaces 2019, 6 (4), 1801627. https://doi.org/10.1002/admi.201801627.[3] Phys. Rev. Lett. 2018, 120 (11), 116602. https://doi.org/10.1103/PhysRevLett.120.116602. Figure caption. The capacitance at 20 Hz and current efficiency at 100 nit are plotted against the ratio of annealing temperature (T) to glass transition temperature (Tg) of ETL. Figure 1

(Invited) Mesoporous Metallic Materials Electrodeposited from Polymeric Micelle Assemblies for Energy Applications

The surfactant-assisted electrodeposition is an effective means to obtain high-quality mesoporous materials in a rather simple synthesis approach. Commercial or home-made block copolymers can be dissolved in aqueous media above their critical micelle concentration to serve as a soft template in the so-called electrodeposition from polymeric micelle assemblies or micelle-assisted electrodeposition in short [1]. The nanoscale porosity of the resulting films, which extends throughout the whole surface and entire film thickness, endow them with large surface-to-volume (S/V) ratios. Such large S/V ratios are particularly amenable for (electro)catalytic applications and, whenever the alloy presents ferromagnetic properties, for magnetoelectric purposes. In our group, mesoporous Cu-Ni, Co-Pt, Ni-Pt and Fe-Pt alloys with pore size around 10 nm have been electrodeposited from Pluronic P-123 containing solutions [2-4]. Similarly, mesoporous Ni films with larger pore diameters (25 - 600 nm) have been deposited using custom-made PS-b-P4VP block copolymer micelles [5]. Even, the double template approach can be utilized to obtain, for example, matrices of mesoporous Co-Pt microdisks when the micelle-assisted electrodeposition is performed on optically lithographed substrates [6], or nanoporous Fe-Pd nanowires using anodized alumina and P-123 as hard and soft templates, respectively [7]. The introduction of an equilibration time between electrolyte preparation and deposition has been determined key in some cases to consistently reproduce the films mesoporosity [8].The electrodeposited mesoporous metallic materials exhibit higher electrocatalytic activity towards hydrogen evolution reaction (HER) in either acid or alkaline media compared to their dense counterparts [3]. In addition, their durability is not severely compromised in spite of the much larger surface exposed to the media. On the other hand, modulation of their magnetic properties with voltage is also possible when these films are electrolyte-gated in aprotic polar solvents like propylene carbonate. The intense electric field strength created at the film / electrolyte interface thanks to the built-in electric double layer allows for tuning of the coercivity (HC) or the saturation magnetization upon voltage application by virtue of different, but often concomitant, mechanisms taking place, like charge carrier accumulation and oxygen migration [6,9]. Variations greater than 30% in HC have been achieved.In this talk I will summarize the main synthesis-wise milestones achieved during the research in the field and the benefits brought by mesoporous alloys to electrocatalysis and magnetoelectricity.[1] C. Li et al. Acc. Chem. Res. 51 (2018) 1764[2] J. Zhang et al. ACS Appl. Mater. Interfaces 10 (2018) 14877[3] K. Eiler et al. Appl. Cat. B 265 (2020) 118597[4] E. Isarain‐Chávez et al. ChemSusChem 11 (2018) 367[5] R. Fagotto Clavijo et al. Cat. Today, in press.[6] C. Navarro-Senent et al. ACS Appl. Mater. Interfaces 10 (2018) 44897[7] D. Raj et al. Nanomaterials 13 (2023) 403[8] C. Navarro-Senent et al. Electrochim. Acta 358 (2020) 136940[9] C. Navarro-Senent et al. APL Materials 7 (2019) 030701

Charge Selective Contacts to Metal Halide Perovskites Studied with Photoelectron Spectroscopy: X‐Ray, Ultraviolet, and Visible Light Induced Energy Level Realignment

AbstractThe electronic properties of metal halide perovskites (MHPs) are crucial for achieving the full potential of MHPs‐based optoelectronic devices, and they are extensively studied by photoelectron spectroscopy (PES). However, there are numerous complexities during PES measurements, which can result in inconsistent experimental data or even misinterpretation of results. Here, it is demonstrated that in the presence of charge selective junctions with MHPs, significant electronic energy level realignment can occur during PES measurements due to the sample excitation with high‐energy photons used in PES, amounting up to over 0.8 eV in the present case. This is caused by unbalanced charge carrier accumulation within the perovskite due to the charge‐selective interface. X‐ray photoelectron spectroscopy further reveals that photoexcitation due to bremsstrahlung, as produced in commonly employed twin‐anode lab‐sources, can readily produce sizable photoinduced shifts of core levels of MHPs films, whereas monochromatized X‐ray lab‐sources (irradiation flux reduced by >50 times by eliminating the bremsstrahlung) induce negligible shifts within the range of presently applied anode powers. The data and measurement conditions presented here are intended to enable others to obtain reliable MHP electronic property information from PES measurements.

Size‐Selective Ionic Crosslinking Provides Stretchable Mixed Ionic–Electronic Conductors

AbstractMechanically compliant conductors are of utmost importance for the emerging fields of soft electronics and robotics. However, the development of intrinsically deformable organic conductors remains a challenge due to the trade‐off between mechanical performance and charge mobility. In this study, we report a solution to this issue based on size‐selective ionic crosslinking. This rationally designed crosslinking mediated by length‐regulated oligo(ethylene glycol) pendant groups and metal ions simultaneously improved the softness and toughness and ensured excellent mixed ionic–electronic conductivity in poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate composite materials. Moreover, the added ions remarkably promoted accumulation of charge carriers in response to temperature gradient, thus offering a viable approach to stretchable thermoelectric generators with enhanced stability against humidity.

Size-Selective Ionic Crosslinking Provides Stretchable Mixed Ionic-Electronic Conductors.

Mechanically compliant conductors are of utmost importance for the emerging fields of soft electronics and robotics. However, the development of intrinsically deformable organic conductors remains a challenge due to the trade-off between mechanical performance and charge mobility. In this study, we report a solution to this issue based on size-selective ionic crosslinking. This rationally designed crosslinking mediated by length-regulated oligo(ethylene glycol) pendant groups and metal ions simultaneously improved the softness and toughness and ensured excellent mixed ionic-electronic conductivity in poly(3,4-ethylenedioxythiophene):polystyrene sulfonate composite materials. Moreover, the added ions remarkably promoted accumulation of charge carriers in response to temperature gradient, thus offering a viable approach to stretchable thermoelectric generators with enhanced stability against humidity.

Simulation study of various factors affecting the performance of vertical organic field-effect transistors

Abstract Vertical organic field-effect transistors (VOFETs) can offer a short channel architecture that can further enhance the performance at low operating voltages, which makes it more viable for organic electronics applications. VOFETs can be prepared using low-cost techniques that reduce the high processing costs and can operate at high current densities and relatively high fRequencies. To further improve the performance, high current density, and operating frequency, the physics of charge carrier transport should be understood well with the simulation. The main problem with VOFET is the high off-current which is inevitable due to conduction from the source to the drain contact. Many efforts have been made to reduce the off-state current by the addition of an insulating layer on top of the source electrode, which further increases the processing complexity and cost of fabrication. Simulations based on device geometry, contact barriers, and organic semiconductor parameters are carried out to study the charge carrier transport in VOFET. The simulation results show that the most important factor, to enhance the performance is the device geometry or architecture, which requires a specific fill factor, a ratio of the exposed gate dielectric width to the total width of the device with the source electrode. The simulation results also show a different type of working physics of the basic VOFET architecture where the On/Off ratio and subthreshold swing are largely dependent on the initial negative gate field instead of the accumulated charge carriers at positive gate fields. Optimized VOFET architecture is then simulated for variation in contact barrier and semiconductor properties, which show further enhancement in performance.

Influence of irradiation energy and damage coefficient on performance of CH3NH3PI3 photocell

An organic–inorganic hybrid perovskite as a light-absorbing layer has become a key material in many laminated-structures used in photon sensing devices. Some of these devices are referred to as Organic–inorganic hybrid perovskite (OIHP)-based devices. In recent days, OIHP applications have sky-rocketed in the world of flexible photo-electronics. Due to the higher radiation tolerances, OIHP-based solar cells have been recommended for terrestrial applications which is an application that require low fabrication costs that combines lightweight materials. However, it has been noticed that when it is used in extra-terrestrial environments, these photo cell devices experience premature failures once they interact with fast multispectral radiations in terrestrial spaces. Due to these premature failures, a similar solar cell from an (OIHP)-based perovskite material of methyl ammonium lead iodide (CH3NH3PI3) was investigated as an observer layer under a multi-spectral simulated illumination. After simulation, data for the expected photocurrent flowing through junction components on a glass substrate coated with a thin layer of Titanium dioxide (TiO 2) film was obtained. The influence of energy of irradiation on damage coefficients, current density, photovoltage and I–V characteristic profile curves were determined and investigated. Some general common macroscopic parameters for photon sensing were analyzed on three dimensions (3D) to determining the minority charge carriers and their induced photo-voltages with respect to junction recombination velocities. The obtained computed macroscopic parameters were incorporated into the Quite Universal Circuit Simulator software for simulation. It was established that damage coefficient influenced the I–V curve and an accumulation of charge carriers magnifies the probability of plasmons initiating degradation of the absorber layer. The atoms in the CH3NH3Pl3 crystal lattice system uniformly recoils at resonance and transport the maximum charge carriers across the P-N junction capable of creating a significant damage concentrated within a few micrometer ranges on the surface that may even extend between 0.39 to about 1.01 micrometers in size. This paper therefore evaluates the influence of irradiation energy and damage coefficient in a hybrid CH3NH3PI3 mono-facet crystal structure on exposure to multi-spectral illumination at varied irradiation energies.

Impact of Interface Energetic Alignment and Mobile Ions on Charge Carrier Accumulation and Extraction in p‐i‐n Perovskite Solar Cells

AbstractUnderstanding the kinetic competition between charge extraction and recombination, and how this is impacted by mobile ions, remains a key challenge in perovskite solar cells (PSCs). Here, this issue is addressed by combining operando photoluminescence (PL) measurements, which allow the measurement of real‐time PL spectra during current–voltage (J–V) scans under 1‐sun equivalent illumination, with the results of drift‐diffusion simulations. This operando PL analysis allows direct comparison between the internal performance (recombination currents and quasi‐Fermi‐level‐splitting (QFLS)) and the external performance (J–V) of a PSC during operation. Analyses of four PSCs with different electron transport materials (ETMs) quantify how a deeper ETM LUMO induces greater interfacial recombination, while a shallower LUMO impedes charge extraction. Furthermore, it is found that a low ETM mobility leads to charge accumulation in the perovskite under short‐circuit conditions. However, thisalone cannot explain the remarkably high short‐circuit QFLS of over 1 eV which is observed in all devices. Instead, drift‐diffusion simulations allow this effect to be assigned to the presence of mobile ions which screen the internal electric field at short‐circuit and lead to a reduction in the short‐circuit current density by over 2 mA cm−2 in the best device.

Tubular Diamond as an Efficient Electron Field Emitter

Herein, we present a straightforward and cost-effective procedure for producing conductive diamond tubes on the surface of porous carbon nanotube hollow fibers using successive 10-pulsed laser annealing shots and 6 h of hot filament chemical vapor deposition techniques. Room-temperature Raman and X-ray diffraction spectra reveal the signature T2g peaks near 1332.4 cm–1 and 111 planes of diamonds near 43.9°, respectively. A low turn-on field (ETO) ∼1.85 V/μm@1 μA/cm2 and a threshold field (ETH) ∼2.54 V/μm@10 μA/cm2 were observed for the tubular diamond structures. The field enhancement factor (β) was calculated at 3594 and highly stable field emission current stability was observed over a long period of 4 h. For the first time, a good insight into the field emission results of the diamond is established with the structural, electronic properties, and the work function (φ) ∼4.84 eV analysis conducted by the density functional theory simulation. Finite electronic states at the Fermi level are observed beyond a band gap, and it demonstrates the wide-band gap (4.4 eV) semiconducting nature of the diamond. The Bader charge analysis and maximum entropy method pattern revealed the negative electron affinity of the diamond, and it is responsible for the emission of electrons from the conduction band of the diamond. Besides, the accumulation of charge carriers, which contributes to the electric field emission, takes place due to the weak π bonds of carbon atoms. The low turn-on field, the high field enhancement factor, and the good field emission current stability of tubular diamond offer great prospects for future efficient and low-cost field emission devices.

Unravelling Dynamics Involving Multiple Charge Carriers in Semiconductor Nanocrystals.

The use of colloidal nanocrystals as part of artificial photosynthetic systems has recently gained significant attention, owing to their strong light absorption and highly reproducible, tunable electronic and optical properties. The complete photocatalytic conversion of water to its components is yet to be achieved in a practically suitable and commercially viable manner. To complete this challenging task, we are required to fully understand the mechanistic aspects of the underlying light-driven processes involving not just single charge carriers but also multiple charge carriers in detail. This review focuses on recent progress in understanding charge carrier dynamics in semiconductor nanocrystals and the influence of various parameters such as dimension, composition, and cocatalysts. Transient absorption spectroscopic studies involving single and multiple charge carriers, and the challenges associated with the need for accumulation of multiple charge carriers to drive the targeted chemical reactions, are discussed.

Layered SrTiO3/BaTiO3 composites with significantly enhanced dielectric permittivity and low loss

Ferroelectric ceramic materials are receiving increasing attention due to their extensive applications in electrical systems. In this work, three-layer BaTiO3/SrTiO3 ceramic composites consisting of one Ba1-xTiO3-SrxTiO3 (at x = 5, 10, 20 and 30) composite interlayer sandwiched by two BaTiO3 outer layers were fabricated. And the effects of SrTiO3 modification on their phase structures, microstructures and dielectric performance were studied in detail. A pure perovskite phase was observed in the Ba1-xTiO3-SrxTiO3 ceramic composites. The BaTiO3 ceramic doping by SrTiO3 exhibited refined grain size. With increased SrTiO3 content, both the permittivity and dielectric loss have increased. Furthermore, a layered structure was designed to enhance the dielectric properties. Consequently, a high permittivity of 7118 and low loss of 0.028 were achieved at 10 kHz in BaTiO3-30 w% SrTiO3–BaTiO3 three-layer ceramic composites. In summary, the introduction of SrTiO3 is believed to reduce the BaTiO3 Curie temperature and greatly improve its permittivity at room temperature. Furthermore, the accumulation of charge carriers at the interface significantly increases the permittivity, while preventing the transport of the charge carriers and reducing the loss. The ultrahigh permittivity and low loss make these composites promising candidates for microwave antennas, field effect transistors and dielectric capacitors.

Spark plasma sintered Si(Hf)OC nanocomposites exhibiting thermally stable dielectric behavior processed through precursor route

This work uses hafnium n tetra butoxide precursor to modify silicon oxycarbide (SiOC) with an easy, novel, and economical procedure to synthesize Si(Hf)OC nanocomposites. FTIR analysis confirmed the formation of Si–O-Hf linkages during cross-linking. The as-pyrolyzed powders were sintered using the spark plasma sintering technique. X-ray diffractograms confirmed the in-situ crystallization of tetragonal HfO2 in an amorphous SiOC matrix. The morphological features were imaged using electron microscopy. The dielectric properties of sintered Si(Hf)OC nanocomposites were investigated at varied temperatures (25 °C–250 °C) and frequency ranges (10−1 Hz–106 Hz) for the first time. At room temperature, the ε՚ value at 1 kHz is found to be 14, which is almost double the value of pure sintered SiOC (ε՚∼5). The dielectric loss (tanδ) is less than 0.07 at 1 kHz at room temperature, which is significantly less than sintered SiOC (tanδ ∼0.5). An increase in permittivity is associated with high permittivity t-HfO2 nanocrystals and additional interfacial/space charge polarization at the interfaces between the t-HfO2 and SiOC matrix. At high-frequency regions (>1 kHz), the dielectric properties of nanocomposites were found to be thermally stable (up to 250 °C). At low-frequency regions (<70 Hz), the material exhibits a rise in dielectric permittivity with increasing temperature, most likely due to charge carrier accumulation at the interfaces of t-HfO2, SiO2, and carbon in the SiOC matrix through the migration of thermally activated charge carriers.