Charge Drift Mobility Research Articles

Dielectric and electrical study of zinc copper ferrite nanoparticles prepared by exploding wire technique

The experimental investigation of electrical and dielectric characteristics of spinel Zn–Cu ferrite (ZCFO) nanoparticles, synthesized adopting an exploding wire technique (EWT)-based approach, is reported in this paper. The direct current (DC) electrical parameters of prepared nanoparticles were studied by two-probe method in the range of 300 K (RT) to 423 K. The activation energy for hopping of charge carriers, drift mobility and charge concentration was determined through DC analysis. The attributes of dielectric behavior like dielectric constant (e′), loss tangent (tan δ), dielectric loss (e″) and alternating current (AC) conductivity (σAC) were measured at various temperatures in the frequency range of 10 Hz–10 MHz. The increase in AC conductivity with frequency, observed in our study, represented the normal behavior of spinel ferrite. The two-layer model based on space charge polarization could satisfactorily elucidate variation in dielectric constant with frequency. The dielectric parameters at different frequencies were also determined in temperature range of 300–673 K. The decrease in DC resistivity as well as the increase in AC conductivity, with rise in temperature, affirms the semiconducting nature of prepared nanoferrite with the band gap energy of 3.16 eV, as calculated through UV–visible analysis. The capacitance of thin grain boundary region (Cgb), grain boundary resistance (Rgb) and relaxation time (τg) of zinc–copper nanoferrite were estimated through Cole–Cole plot analysis.

Transient Photocurrent Elucidating Carrier Dynamics and Potential of Bulk Heterojunction Solar Cells Fabricated by Thermal Precursor Approach

Transient photocurrents are measured to clarify carrier dynamics in a series of bulk heterojunction solar cells that are composed of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and diketopyrrolopyrrole‐tetrabenzoporphyrin conjugates substituted by straight alkyl groups (Cn‐DPP‐BP), of which the device parameters are largely dependent on the length of the substituted alkyl groups (Cn). The time profile of the observed transients indicates the presence of both unimolecular and bimolecular charge recombination processes in the active layer of the cells. Analysis of the photocurrent decays detected at various bias voltages using a drift model that considers charge extraction by electrodes and charge recombination in the active layer enables elucidation of the charge drift mobility, the initial photocarrier density, and the charge recombination rate constants. The bimolecular recombination rate constant is correlated with Cn and this is the main factor that determines the power conversion efficiency (η) of the Cn‐DPP‐BP:PCBM solar cells. The Cn‐dependent fill factor can be interpreted in terms of the bias voltage dependence of the ratio between the extraction and recombination charges (Qex/Qre). Comparison of η and Qex/Qre at the short‐circuit condition predicts that η for the Cn‐DPP‐BP:PCBM solar cell can be potentially improved up to 6%.

Benzo[b]carbazole and indole derivatives as emitters for non-doped deep-blue organic light emitting diodes

Derivatives of indole and benzo[b]carbazole were prepared by employing efficient regioselective acid-catalysed synthesis of 1H, 1-methyl and 1-naphthylindole with o-phthaldialdehyde. The synthesized compounds were characterized by NMR, IR and mass spectrometries. They form molecular glasses with glass transition temperatures ranging from 86 to 116 °C. Naphthyl-substituted derivatives showed high thermal stabilities with 5% weight loss temperatures of 409 and 413 °C, while their methyl-substituted counterparts exhibited by ca. 90 °C lower 5% weight loss temperatures. Their solutions exhibited blue emission with quantum yields of 67%. Ionization potentials of the compounds estimated by cyclic voltammetry are in the range of 5.49–5.65 eV, while the values obtained using photoelectron emission in air method are in the range of 5.16–5.28 eV. The layers of the compounds exhibited ambipolar charge transport with time-of-flight charge drift mobility values well exceeding 10−3 cm2/(Vs) at high electric fields. Non-doped naphthylindole and benzo[b]carbazole derivative based device emitted deep-blue light with the maximum intensity at 410 nm and CIE of (0.21, 0.16), and it exhibited a maximum external quantum efficiency of 3.4%.

Diphenylamine‐Substituted Carbazole‐Based Hole Transporting Materials for Perovskite Solar Cells: Influence of Isomeric Derivatives

AbstractA series of new branched hole transporting materials (HTMs) containing two diphenylamine‐substituted carbazole fragments linked by a nonconjugated methylenebenzene unit is synthesized and tested in perovskite solar cells. Synthesis of the investigated materials is performed by a simple two‐step synthetic procedure providing a target product in high yield. The isolated materials demonstrate good thermal stability and majority of the investigated compounds exist in an amorphous state, which is advantageous as there is no risk of crystallization directly in the film. The highest charge drift mobility of µ0 = 4 × 10−4 cm2 V−1 s−1, measured at weak electric fields, is by ca. one order of magnitude higher than that of Spiro‐OMeTAD under identical conditions. From the perovskite solar cell testing results, it can be seen that performance of two new HTMs (V885 and V911) is on a par with Spiro‐OMeTAD. Due to the ease of synthesis, good thermal, optical and photophysical properties, this type of molecules hold great promise for practical application in commercial perovskite solar cells.

The density-of-states contributions to the negative field charge drift mobility effect in poly(3-hexylthiophene) organic semiconductor

The origin of the empirical linear dependence of Eint the electric field at the charge-injecting metal/poly(3-hexylthiophene), P3HT, interfaces on the externally applied bias, Ea, that governs its room temperature negative field charge drift mobility is investigated. The published electrostatic model is modified in the sense that the energetic shift φ of the Gaussian disordered hole and electron energy states is determined to be a linear function of the externally applied electric field, Ea. On this basis the distorted Gaussian shaped interfacial electric field Eint is obtained and the empirical Eint is identified as the linear interpolation function of positive slope through the inflection point of the calculated curve. The disordered energy states extend throughout the P3HT charge transport gap and the populations of interface charges follow unequal Ea dependences. The coupling between the deduced interfacial electric field Eint and the P3HT effective mobility enables the predictions to be compared to the published time-of-flight room temperature negative field drift mobility data of holes and electrons in various metal/P3HT sandwich-type organic structures.

Dielectric response and room temperature ferromagnetism in Cr doped anatase TiO2 nanoparticles

In the present work, nanocrystalline samples of Ti1−xCrxO2 (x=0, 0.02, 0.04, 0.06 and 0.08) were synthesized in anatase phase through simple and cost effective acid modified sol gel method. The influence of Cr doping on thermal, microstructural, electrical and magnetic properties was investigated in TiO2 host matrix. The surface morphology has revealed less agglomeration and considerable reduction in particle size in case of Cr doped TiO2 as compared to undoped TiO2 nanoparticles (NPs). Energy dispersive x-ray spectroscopy (EDS), Raman and X-ray photoelectron spectroscopy (XPS) established high purity, appropriate stoichiometry and oxidation states of the compositions. The dielectric properties of the nanoparticles were altered by the doping concentration, applied frequency as well as temperature variation. The variation in dielectric constant (ε′), dielectric loss (δ) and ac conductivity as a function of frequency and temperature at different doping concentration of Cr were interpreted in the light of Maxwell Wagner theory, space charge polarization mechanism and drift mobility of charge carriers. Both undoped and Cr doped TiO2 samples exhibit room temperature ferromagnetism (RTFM) that remarkably influenced by means of the Cr content. The significant enhancement in the magnetization was observed at 4% Cr doping. However, decrease in magnetization for higher doping signify antiferromagnetic interactions between Cr ions or superexchange mechanism. These results reveal that the oxygen vacancies play a crucial role to initiate the RTFM. Therefore, the present investigation suggests the potential applications of Cr doped TiO2 nanoparticles for spintronics application.

The impact of neutral impurity concentration on charge drift mobility in n-type germanium

The impact of neutral impurity scattering of electrons on the charge drift mobility in high purity n-type germanium crystals at 77 Kelvin is investigated. We calculated the contributions from ionized impurity scattering, lattice scattering, and neutral impurity scattering to the total charge drift mobility using theoretical models. The experimental data such as charge carrier concentration, mobility and resistivity are measured by Hall Effect system at 77 Kelvin. The neutral impurity concentration is derived from the Matthiessen's rule using the measured Hall mobility and ionized impurity concentration. The radial distribution of the neutral impurity concentration in the self-grown crystals is determined. Consequently, we demonstrated that neutral impurity scattering is a significant contribution to the charge drift mobility, which has a dependence on the concentration of neutral impurities in high purity n-type germanium crystal.

The impact of neutral impurity concentration on charge drift mobility in p-type germanium

We reported a new result of the neutral impurity scattering of holes that has impact on the charge drift mobility in high purity p-type germanium crystals at 77 Kelvin. The charge carrier concentration, mobility and resistivity are measured by Hall Effect system at 77 Kelvin. We investigated the contribution to the total charge drift mobility from ionized impurity scattering, lattice scattering, and neutral impurity scattering with the best theoretical models and experimental data. Several samples with measured Hall mobility from the grown crystals are used for this investigation. With the measured Hall mobility and ionized impurity concentration as well as the theoretical models, we calculated the neutral impurity concentration by the Matthiessen's rule. As a result, the distributions of the neutral impurity concentrations with respect to the radius of the crystals are obtained. Consequently, we demonstrated that neutral impurity scattering is a significant contribution to the charge drift mobility, which has dependence on the concentration of neutral impurities in a given germanium crystal.

Dielectric and magnetic investigations of mixed cubic spinel Co-ferrites with controlled Mg content

High temperature frequency dependent dielectric properties, and room temperature magnetic behavior of mixed ferrites with controlled content of Mg in Co 1−xMg xFe 2O4(x=0.0,0.1,0.3,0.5,0.7,0.9 and 1.0) compositions are studied. Single phase spinel structure with cubic symmetry, lattice parameters, crystallite size, magnetic and dielectric properties were substantiated with x-ray diffractometer, transmission electron microscope, vibrating sample magnetometer and impedance analysis, respectively. Due to interfacial polarization, dielectric behavior of all the compositions shows dispersion with increase in frequency. The dielectric data was investigated by comparing the tangent loss and electric modulus for assigning the type and mechanism of dielectric relaxation. Temperature dependent dielectric constant, tangent loss and AC conductivity increase due to thermal activation of charge carriers and drift mobility. Furthermore, room temperature weak ferromagnetic behavior is observed due to the incorporation of non-magnetic Mg ions.

Phenylethenyl‐Substituted Triphenylamines: Efficient, Easily Obtainable, and Inexpensive Hole‐Transporting Materials

Star-shaped charge-transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one-step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glass-transition temperature, size of the π-conjugated system, energy levels, and the way molecules pack in the solid state can be significantly influenced by variation of the structure of these side arm(s). An increase in the number of phenylethenyl side arms was found to hinder intramolecular motions of the TPA core, and thereby provide significant enhancement of the fluorescence quantum yield of the TPA derivatives in solution. On the other hand, a larger number of side arms facilitated exciton migration through the dense side-arm network formed in the solid state and, thus, considerably reduces fluorescence efficiency by migration-assisted nonradiative relaxation. This dense network enables charges to move more rapidly through the hole-transport material layer, which results in very good charge drift mobility (μ up to 0.017 cm(2) V (-1) s(-1)).

Thickness dependence of the trap states in organic thin film of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine

The authors have investigated the relationship between the trap states (exponential trap distribution in energy and density) and the thickness of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB). The thickness dependent hole mobility of NPB can be attributed to the trap states. The origin of deep trap states at thinner film can be attributed to both surface dipole of buckminsterfullerene and the interaction between NPB and indium tin oxide at the interface. The influence of interfacial trap states on charge drift mobility is getting weaker as the thickness increases and is negligible when the thickness of NPB is thicker than 300nm.

IBIC characterization of charge transport in CdTe:Cl

Studies of charge transport uniformity in bulk CdTe:Cl have been carried out using ion-beam-induced charge (IBIC) imaging. High resolution maps of charge collection efficiency, mobility-lifetime product (μτ), and drift mobility (μ) were measured using a scanning microbeam of 2 MeV protons focused to a beam diameter of ∼3 μm. Excellent charge transport uniformity was observed in single crystal CdTe:Cl, with electron μτ values of up to 5 × 10−3 cm2/V s. The presence of extended defects such as tellurium inclusions was also studied using IBIC, and their influence on the charge transport performance of CdTe detector structures is discussed.

The role of ionic conductivity and interface in electrical resistance, ion transport and transmembrane redox reactions through polyaniline membranes

Three different physico-chemical properties of HCl-doped polyaniline (PANI) membranes, separating two aqueous solutions are compared. The first one is electrical conductivity measured with electrochemical impedance spectroscopy (EIS). The second one is H + ion permeation through the membrane under pH gradient, measured with pH electrodes, and the third is electron/ion coupled counter transport in a transmembrane redox reaction, measured with redox electrodes. Electrophysical properties of undoped membrane are not described by the Ohm's law. The non-linear dependence is due to a limited content of mobile electrons and is similar to the Child's law known for vacuum diodes and dielectrics. Although the impedance of the doped membrane separating aqueous solutions is much less than that for the undoped membrane, it is determined by ions and not by electrons. For doped membranes the ratio of ion diffusion coefficient and charge drift mobility, determined in the external electrical field, is close to the Einstein relationship, meaning that the same ions play the key role in both processes. Thickness dependent behaviour is used to differentiate the interfacial properties from those of the bulk properties of the membrane. Interpolation of properties to zero membrane thickness shows that the interfacial charge transport plays an important role in doped membrane impedance and influences ion and redox transport rates. The relative role of interface versus volume increases with acid doping, which makes the bulk membrane volume more permeable for ions.

Spin diffusion of a two-dimensional electron gas in the random phase approximation

Spin transport properties of spin-polarized two-dimensional electron gas are studied in the presence of electron–electron interactions. Longitudinal and transverse spin diffusion coefficients are calculated with the quantum transport equation. New results are obtained by evaluating both the drift and the collision terms using the random phase approximation, while the drift term was calculated by Hartree–Fock approximation in our earlier work. The qualitative features remain same, but the present work is valid for larger polarizations. We find that the e–e scattering, which does not contribute to the charge drift mobility, has a significant contribution to the spin diffusion.

Electron transport in a starburst oxadiazole

In this article we report on the electron mobility of a starburst oxadiazole, 1,3,5 -[(4-tert.-butylphenyl)-2-oxadiazolyl] benzene. For direct evidence of the electron transport capability the charge drift mobility and its field dependence have been measured by the time-of-flight technique. The electron mobility showed a square root dependence of the electric field and was somewhat larger than that for PBD. In addition, the oxadiazole compound has been used as the electron transport layer (ETL) in light-emitting diodes (LEDs) together with poly(1,4-phenylene vinylene). The characteristics of these devices are compared to analogous LEDs with Alq3 as the ETL.

Electronic Conduction in Poly(p-phenylene-1,3,4-oxadiazole) Films

Dark conductivity and photoconductivity in poly(p-phenylene-1,3,4-oxadiazole) (POD) films were investigated. Above 440 K, ionic conduction was observed. An electronic process which was delicately influenced by the presence of adsorbed oxygen was found below 440 K. Large photoconductivity was observed and its wavelength and temperature dependences were examined. POD films were pyrolyzed at the temperature between 670 and 810 K to give semiconductive polymer films with the conductivity between 10−20 and 10−5 S cm−1. Variations of dark conductivity, photoconductivity and charge drift mobility with pyrolysis were investigated. Charge drift mobilities of pyrolyzed POD films were around 10−7 cm2V−1s−1 and moderately dependent on the pyrolysis temperature.

On the reversible change in the electrical conductivity of a polyimide during pulsed irradiation

A kinetic study has been made of the attainment and decay of electrical conductivity excited in a polyimide irradiated with fast electron pulses (particle energy 8 MeV, 40 nsec and 2 μsec pulses). It has been found that the delayed component of the radiation induced electrical conductivity consists of fast (τ 1 2 =150 nsec) and slow (τ 1 2 =2 μ sec) components. It is proposed that the fast component is related to a rearrangement of captured charges from shallow to deeper traps, while the slow component is responsible for the experimentally observed inhomogeneous character of the recombination of charge carriers created in the polyimide by irradiation. It is shown that relaxation of induced electrical conductivity after the cessation of irradiation is not a function of the electric field voltage. An estimate of the captured charge drift mobility at room temperature gives μ dr=3·6×10 −7 cm 2/W·sec.

Charge Mobility of Organic Semiconductors under High Pressure. Anthracene

Abstract The charge mobilities of anthracene single crystals were measured under high pressures up to 7.4 kbar. An experimental apparatus like Kepler’s was used. A sample holder was attached in a Drickamer-type high-pressure photocell, with kerosene employed as the pressure-transmitting fluid. The electron mobilities along the a and b crystal axes increased as the pressure applied increased; the values of the charge drift mobility at 7 kbar were 1.7 times as large as those at atmospheric pressure. The electron mobility along the c′ axis was nearly constant. The hole mobility, however, could not be measured, because the tail of the pulse signal, observed on the oscilloscope, was deformed.