Microwave Conductivity Research Articles

Anomalous Dielectric Behavior of a Pb/Sn Perovskite: Effect of Trapped Charges on Complex Photoconductivity

Organic–inorganic metal halide perovskites (MHPs) exhibit prominent electronic and optical properties benefiting the performance of solar cells and light-emitting diodes. However, the dielectric properties of these materials have remained poorly understood, despite probably influencing delayed charge recombination and device capacitance. Herein, we characterize the unprecedented dielectric behavior of MHPs comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides using time-resolved microwave conductivity (TRMC) measurements. At specific compositions, the above MHPs exhibit negative real and positive imaginary photoconductivities, the polarities of which are opposite those observed for conventional photogenerated charge carriers. Comparing the observed TRMC kinetics with that of inorganic perovskites (SrTiO3 and BaTiO3) and characterizing its dependence on temperature, frequency, and near-infrared second push pulse, we conclude that the above behavior is due to the trapping of polaronic hole...

Peripherally Cyanated Subphthalocyanines as Potential n-Type Organic Semiconductors.

A series of peripherally dicyano-, tricyano-, and tetracyano-substituted subphthalocyanines (SubPcs) have been prepared through microwave-assisted, palladium-mediated cyanation of iodinated precursors. The introduction of π-accepting cyano groups in the macrocycle clearly influences its electronic and redox properties, which are dependent on the number and relative position of these substituents. Additional functionalization of the periphery of SubPcs with electron-donating or -withdrawing groups allows for a further fine-tuning of their features, leading to intensely absorbing, strongly electron-accepting panchromatic dyes with low-lying LUMO energy levels. Flash-photolysis time-resolved microwave conductivity measurements on vapor-deposited films demonstrate that some of these novel SubPc derivatives display remarkable intrinsic charge-carrier mobilities that are comparable to or larger than those of other known well-performing acceptor SubPcs; thus confirming their potential as n-type organic semiconductors for application in the fabrication of photovoltaic devices.

Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces

We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds timescales using time-resolved terahertz spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ∼150 nm crystallites reaches 750 cm2/V s, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO in...

Relaxation of Plasma Carriers in Graphene: An Approach by Frequency‐Dependent Optical Conductivity Measurement

AbstractThe relaxation time and mobility of electrons on graphene are deduced quantitatively by complex permittivity analysis with microwave dielectric loss spectroscopy. Hexagonal graphene sheets with uniform domain sizes from 6 to 30 µm, which target the mean free path of electrons on graphene based on the significantly longer turnover time of microwave probing at gigahertz (GHz) frequencies as compared to the relaxation time of electrons, are employed to determine the factors for carrier relaxation at the intradomain and grain boundaries. The changes in the complex permittivity by carrier injections clearly show an evidence of plasma carriers in graphene. This result validates the plasma carrier model with the restoring force in the GHz regime instead of the free carrier model, which is applied to analyze the optical conductivity over the terahertz (THz) regime. Based on Dirac massless fermions in graphene, the relaxation time of electrons is estimated to be 250–450 fs from the changes in complex permittivity, which are experimentally determined by frequency‐dependent microwave conductivity measurements. The obtained relaxation time shows a positive correlation with the domain size of graphene, implying that the scattering dominantly occurs at the domain boundary.

Drude-jellium model for the microwave conductivity of electrolyte solutions

The microwave conductivity characteristics of electrolyte solutions have attracted much interest of researchers because a good understanding of their properties plays a key role to study fundamental processes in biology and chemistry. In this work, we consider the solution of sodium chloride as a plasma consisting of ions with water background. Its plasmon frequency is calculated by the jellium theory. The linear dependence of the microwave conductivity on the ion concentration of the electrolyte solutions is explained by a microscopic approach and described by a combination of this plasmon relationship and the simplified Drude formula for dielectric constant. Furthermore, the dependence of the microwave conductivity on the frequency of the salt solution is also examined. We suggest that it obeys the logistic distribution. We found a good agreement between theoretical calculations and experimental data. The values of the damping coefficient γ for the conductive solutions at low frequencies and the cutting frequency are estimated. The linear dependence of the diffusion coefficient on the temperature of the salt solution is also shown, in similarity with the result in the other model. The application of the Drude-jellium model could be done for the other electrolyte solutions in order to study theirs electro-dynamic properties.

Dynamic Microwave Conductivity of Graphene-Based Shungite

The conductivity of thin graphene-based shungite sheets has been experimentally investigated. Direct-current measurements have been performed upon reflection of microwaves. It is shown that, at a low carbon content, the dynamic conductivity of shungite is similar to its static conductivity and, at a high carbon content, exceeds the latter by more than an order of magnitude. The data obtained are interpreted using shungite structural components.

(Invited) Configuration of Organic Conjugated Molecular/Polymer Systems at the Interfaces and Under High Pressure: Non-Contact Assessment By Microwave Electrical Conductivity Measurements

Tuning and optimization of charge carrier transport at interfaces is one of the most important subjects in the design of opto-electronic devices and materials. Charge carriers are injected or extracted through metal/semiconductor interfaces in most electronic devices, while carrier transport occurs at insulator/semiconductor interfaces rather than in the bulk in the major organic electronic devices.1,2 However, analytical techniques for evaluating such interfacial carrier transport phenomena are still limited, and this remains a challenging issue. We have recently reported a technique, referred to as time-resolved microwave conductivity (TRMC)3, and the system has been extended into field-induced TRMC that combines charge carrier injection via gate bias applied into working devices and microwave-based non-contact probing of intrinsic and local charge carrier motion. Using this technique, hole and electron mobilities were separately and quantitatively determined in a series Au/organic semiconductor/PMMA/SiO2/Au MIS devices,4,5 demonstrating that the non-contact, fully experimental evaluation of intra-domain carrier mobility at interfaces is quite unprecedented and is a characteristic feature of this system. Herein, we further report that the FI-TRMC technique can distinguish between mobile charge carriers at the interface and immobile charges trapped at defects, thus enabling quantification of both the charge carrier mobility and the density of trap sites at insulator-semiconductor interfaces. References G. Horowitz and P. Delannoy, J. Appl. Phys. 70, 469 (1991).H. Klauk, Chem. Soc. Rev. 39, 2643 (2010).S. Seki, et al., Phys. Chem. Chem. Phys. 16, 11093 (2014); Acc. Chem. Res. 45, 1193 (2012); Nature Commun. 5, 3718 (2014); Nature Commun. 4, 2694 (2013); Nature Commun. 4, 1691 (2013)Y. Honsho, T. Miyakai, T. Sakurai, A. Saeki, and S. Seki, Sci. Rep. 3, 3182 (2013).W. Choi, Y. Tsutsui, S. Seki, et al., Appl. Phys. Lett. 105, 019430 (2014); Adv. Mater. 28, 7106 (2016); Appl. Phys. Lett. 110, 153303 (2017); Appl. Phys. Lett. 110, 203302 (2017)

Durian-Shaped CdS@ZnSe Core@Mesoporous-Shell Nanoparticles for Enhanced and Sustainable Photocatalytic Hydrogen Evolution.

In artificial photosynthesis, the establishment of design guidelines for nanostructures to maximize the photocatalytic performance remains a great challenge. In contrast with the intense research into band-offset tuning for photocatalysts, the relationship between nanostructures and photoinduced carrier dynamics has still been insufficiently explored. We synthesized durian-shaped CdS@ZnSe core@mesporous-shell nanoparticles ( d-CdS/ZnSe NPs) and investigated the carrier dynamics in photocatalytic hydrogen evolution. The cocatalyst-free d-CdS/ZnSe NPs exhibited high photocatalytic activity for H2 evolution (14.8% apparent quantum yield at 420 nm) and excellent stability (maintaining 80% activity after 72 h) under visible-light irradiation (>422 nm). The transient absorption measurement and flash photolysis time-resolved microwave conductivity unveiled that the ultra-long-lived charge separation (>6.2 ms) and swift hole transfer to the surfaces of ZnSe shell (11 ns) contribute the high catalytic activity and stability. The present work provides a novel insight into designing nanoparticulate photocatalysts with optimized performance.

Lifetime Enhancement of Visible Light Induced Photocharges in Tungsten and Nitrogenin situCodoped TiO2:WN Thin Films

We report on one-step in situ codoped TiO2 thin films synthesized by cosputtering. The purpose of this acceptor–donor passivated codoping approach is to overcome the optoelectronic limitations that arise for monodoped TiO2 in photocatalytic applications. To evaluate these added benefits, the TiO2:WN thin films were characterized by different techniques. X-ray diffraction patterns and X-ray photoelectron spectral analysis revealed that both N and W dopants are mostly present in the desired substitutional locations. Additionally, the codoping approach was found to reduce the internal strain and defect density of the TiO2:WN films as compared to their monodoped TiO2:N counterparts. This defect reduction is confirmed via photocharge lifetime variation obtained using visible light flash photolysis time-resolved microwave conductivity measurements (FP-TRMC). Photocharge lifetime analysis indicated the presence of three distinct decay processes: charge trapping, recombination, and surface reactions. These charac...

Delocalization Drives Free Charge Generation in Conjugated Polymer Films

We demonstrate that the product of photoinduced electron transfer between a conjugated polymer host and a dilute molecular sensitizer is controlled by the structural state of the polymer. Ordered semicrystalline solids exhibit free charge generation, while disordered polymers in the melt phase do not. We use photoluminescence (PL) and time-resolved microwave conductivity (TRMC) measurements to sweep through polymer melt transitions in situ. Free charge generation measured by TRMC turns off upon melting, whereas PL quenching of the molecular sensitizers remains constant, implying unchanged electron transfer efficiency. The key difference is the intermolecular order of the polymer host in the solid state compared to the melt. We propose that this order–disorder transition modulates the localization length of the initial charge-transfer state, which controls the probability of free charge formation.

Charge Carrier Dynamics in Cs2AgBiBr6 Double Perovskite.

Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs2AgBiBr6 by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm2/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 1016/cm3 in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.

Photocatalytic properties of BiOCl-TiO2 composites for phenol photodegradation

BiOCl-TiO2 composites were synthesized by sol-gel method; using two commercial BiOCl (P2600 and SB) with different BiOCl-TiO2 weight ratios. They were characterized by different techniques such as X-ray diffraction (XRD), electron microscopy (SEM, HRTEM and TEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), time resolved micro-wave conductivity (TRMC) and UV–vis diffuse reflectance spectroscopy (UV-DRS). In addition, these composites (BiOCl-TiO2) were evaluated for the photodegradation of phenol (50 mgL−1) under visible irradiation (λ > 450 nm). The results showed effective phenol degradation with the PTi-75 composite, which has 75% by weight of TiO2, obtaining up to 40% of degradation during 6 h of reaction. The SEM analysis showed that micro-sheets of BiOCl are irregularly embedded on agglomerates of TiO2 nanoparticles. A mechanism was proposed, which considers the excitation by the overlap of the BiOCl-TiO2 bands; where the TiO2 has a conduction band more electronegative than that of BiOCl, allowing that TiO2 electron of the conduction band can be transferred to conduction band of BiOCl; while the holes present in the valence band of BiOCl can be moved to TiO2 valence band preventing the electron-holes recombination.

Investigating charge generation in polymer:non-fullerene acceptor bulk heterojunction films

Non-fullerene acceptors are now capable of being used in high efficiency bulk heterojunction (BHJ) donor-acceptor organic solar cells. Acceptors comprising single or multiple linked chromophores have been used. We have developed a new non-fullerene molecular acceptor as well as two non-polymeric macromolecular materials that contain four equivalents of a similar chromophore, but can adopt different spatial arrangements of the chromophores. We compare the effect of having single and multiple chromophores within a macromolecule on the charge generation processes in P3HT:non-fullerene acceptor BHJ films using Transient Absorption Spectroscopy (TAS) and Time Resolved Microwave Conductivity (TRMC) measurements. It was found from the TAS measurements that at low weight percent (5 wt%) the single chromophore formed more polarons than the acceptors in which chromophores were linked, due to it having a more even distribution within the film. At higher concentrations (50 wt%) the trend was reversed due to the single chromophore forming crystalline domains, which reduced the interface area with the P3HT donor. The TRMC measurements showed that more mobile carriers were formed in the macromolecular acceptors when used at low concentrations in the blend and, independent of concentration, mobile carriers had a longer lifetime when compared to films containing the molecular material, which we ascribe to the charges being able to sample more than one chromophore and thus reduce recombination events.

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States, Charge Transport, and Recombination

AbstractAdding cesium (Cs) and rubidium (Rb) cations to FA0.83MA0.17Pb(I0.83Br0.17)3 hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.

Charge-Carrier Dynamics and Crystalline Texture of Layered Ruddlesden–Popper Hybrid Lead Iodide Perovskite Thin Films

Solution-processable organic metal halide Ruddlesden–Popper phases have shown promise in optoelectronics because of their efficiencies in solar cells along with increased material stability relative to their three-dimensional counterparts (CH3NH3PbI3). Here, we study the layered material butylammonium methylammonium lead iodide (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 for values of n ranging from 1 to 4. Thin films cast from solution show a gradual change in the crystalline texture of the two-dimensional lead iodide layers from being parallel to the substrate to perpendicular with increasing n. Contactless time-resolved microwave conductivity measurements show that the average recombination rate order increases with n and that the yield–mobility products and carrier lifetimes of these thin films are much lower than that of CH3NH3PbI3, along with increased higher-order recombination rate constants.

Superconducting fluctuations in FeSe0.5Te0.5 thin films probed via microwave spectroscopy

We investigated the microwave conductivity spectrum of FeSe$_{0.5}$Te$_{0.5}$ epitaxial films on CaF$_2$ in the vicinity of the superconducting transition. We observed the critical behavior of the superconducting fluctuations in these films with a dimensional crossover from two-dimensional to three-dimensional as the film thickness increased. From the temperature dependence of the scaling parameters we conclude that the universality class of the superconducting transition in FeSe$_{0.5}$Te$_{0.5}$ is that of the 3D-XY model. The lower limit of the onset temperature of the superconducting fluctuations, Tonset, determined by our measurements was 1.1 Tc, suggesting that the superconducting fluctuations of FeSe$_{0.5}$Te$_{0.5}$ are at least as large as those of optimally- and over-doped cuprates.

Supramolecular Chirality Issues in Unorthodox Naphthalene Diimide Gelators

Herein, the self-assembly of a few 1,3-dihydroxyl functionalized naphthalene diimide (NDI) derivatives has been reported with particular emphasis on the impact of chirality on gelation and the effect of self-assembly on charge-carrier mobility. A nonconventional gelator (R)-NDI, devoid of any long alkyl chains, exhibited spontaneous gelation in tetra-chloroethylene (TCE). Based on X-ray crystallography and powder X-ray diffraction studies, it was established that a ladder-like hydrogen-bonded chain formation between the 1,3-dihydroxyl group leads to the fibrillar structures with preferential helicity. Likewise the (S)-isomer also exhibited identical gelation and mesoscopic structure but produced fibrils with the opposite handedness. Intriguingly, an equimolar mixture of the (R)- and (S)-isomers did not show any gelation ability, rather a macroscopic precipitation was observed and, in sharp contrast to the individual isomers, the morphology of the mixture showed ill-defined near spherical agglomerates. Differential scanning calorimetry (DSC) studies revealed an identical crystallization peak for the supramolecular polymer produced from the enantiopure gelators ((R)- or (S)-isomer), which was absent in their equimolar mixture. However, mixtures of the two isomers with enantiomeric excess retrieved the gelation ability and preferential helicity demonstrating that chiral amplification is operative in the present system through the so-called "majority rule" effect. Chirality induction was also realized by the "sergeant and soldier" principle in the supramolecular assembly of an achiral NDI gelator in the presence of either the (R)- or (S)-isomer as the chiral dopant. However, the strong helical bias induced by the chiral gelator was found to be opposite in nature compared to that found in the self-assembly of the pure chiral gelator that has been rarely reported in the literature. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements indicated the strong positive impact of the gelation on the electrical conductivity.

Microwave Non-Destructive Inspection and Prediction of Modulus of Rupture and Modulus of Elasticity of Engineered Cementitious Composites (ECCs) Using Dual-Frequency Correlation.

This research article presents dual-frequency correlation models for predicting the growth of elasticity and flexural strength of engineered cementitious composites (ECCs) using microwave nondestructive inspection technique. Parallel measurements of microwave properties and mechanical properties of ECC specimens were firstly undertaken in the sense of cross-disciplinary experiments. Regression models were developed via means of nonlinear regression to the measured data. The purpose of the study is: (i) to monitor the flexural strength and elasticity growth; and (ii) to predict their mature values under the influence of different initial water contents, via microwave effective conductance at early ages. It has been demonstrated that both the modulus of rupture (MOR) and modulus of elasticity (MOE) can be accurately modeled and correlated by microwave conductance using exponential functions. The moduli developed as a function of conductance whereas the regression coefficient exhibited a linear relation with water-to-binder ratio. These findings have highlighted the effectiveness of the microwave non-destructive technique in inspecting the variation of liquid phase morphology of ECCs. The dual-frequency correlation can be used for structural health monitoring, which is not only for prediction but also provides a means of verification.

Enhanced electromagnetic interference shielding properties of carbon fiber veil/Fe3O4 nanoparticles/epoxy multiscale composites

The multiscale approach has been adapted to enhance the electromagnetic interference shielding properties of carbon fiber (CF) veil epoxy-based composites. The Fe3O4 nanoparticles (NPs) were homogeneously dispersed in the epoxy matrix after surface modification by using silane coupling agent. The CF veil/Fe3O4 NPs/epoxy multiscale composites were manufactured by impregnating the CF veils with Fe3O4 NPs/epoxy mixture to prepare prepreg followed by vacuum bagging process. The electromagnetic interference shielding properties combined with the complex permittivity and complex permeability of the composites were investigated in the X-band (8.2–12.4 GHz) range. The total shielding effectiveness (SET) increases with increasing Fe3O4 NPs loadings and the maximum SET is 51.5 dB at low thickness of 1 mm. The incorporation of Fe3O4 NPs into the composites enhances the complex permittivity and complex permeability thus enhancing the electromagnetic wave absorption capability. The increased SET dominated by absorption loss SEA is attributed to the enhanced magnetic loss and dielectric loss generated by Fe3O4 NPs and multilayer construction of the composites. The microwave conductivity increases and the skin depth decreases with increasing Fe3O4 NPs loadings.

Spatial Inhomogeneity of Methylammonium Lead-Mixed Halide Perovskite Examined by Space- and Time-Resolved Microwave Conductivity.

Reducing the spatial inhomogeneity of solution-processed, multicrystalline methylammonium lead iodide (MAPbI3) perovskite is of great importance for improving its power conversion efficiency, suppressing point-to-point deviations, and delaying degradation during operation. Various techniques, such as conducting-mode atomic force microscopy and photoluminescence mapping, have been applied for this intriguing class of materials, revealing nonuniform electronic properties on the nanometer-to-micrometer scale. Here, we designed a new space- and time-resolved microwave conductivity system that enables mapping of the transient photoconductivity with resolution greater than ∼45 μm. We examined the effects of the precursor concentration of MAPbI3 and the mixing of halides (I– and Br–) on the charge carrier dynamics, crystal size, and inhomogeneity of the films. The optoelectronic inhomogeneity of MAPbI3 and MAPb(I1–xBrx)3 on the sub-millimeter and millimeter scales shows a general correlation with their crystallite sizes, whereas the precursor concentration and halide mixing affect the inhomogeneity in a different way, providing a basis for uniform processing of a multicrystalline perovskite film.