Drift Mobility Research Articles

Charged-particle spectroscopy in organic semiconducting single crystals

The use of organic materials as radiation detectors has grown, due to the easy processability in liquid phase at room temperature and the possibility to cover large areas by means of low cost deposition techniques. Direct charged-particle detectors based on solution-grown Organic Semiconducting Single Crystals (OSSCs) are shown to be capable to detect charged particles in pulse mode, with very good peak discrimination. The direct charged-particle detection in OSSCs has been assessed both in the planar and in the vertical axes, and a digital pulse processing algorithm has been used to perform pulse height spectroscopy and to study the charge collection efficiency as a function of the applied bias voltage. Taking advantage of the charge spectroscopy and the good peak discrimination of pulse height spectra, an Hecht-like behavior of OSSCs radiation detectors is demonstrated. It has been possible to estimate the mobility-lifetime value in organic materials, a fundamental parameter for the characterization of radiation detectors, whose results are equal to μτcoplanar = (5 .5 ± 0.6 ) × 10−6 cm2/V and μτsandwich = (1 .9 ± 0.2 ) × 10−6 cm2/V, values comparable to those of polycrystalline inorganic detectors. Moreover, alpha particles Time-of-Flight experiments have been carried out to estimate the drift mobility value. The results reported here indicate how charged-particle detectors based on OSSCs possess a great potential as low-cost, large area, solid-state direct detectors operating at room temperature. More interestingly, the good detection efficiency and peak discrimination observed for charged-particle detection in organic materials (hydrogen-rich molecules) are encouraging for their further exploitation in the detection of thermal and high-energy neutrons.

Absorption and optical conduction in InSe/ZnSe/InSe thin film transistors

In this work, (n)InSe/(p)ZnSe and (n)InSe/(p)ZnSe/(n)InSe heterojunction thin film transistor (TFT) devices are produced by the thermal evaporation technique. They are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion X-ray spectroscopy and optical spectroscopy techniques. While the InSe films are found to be amorphous, the ZnSe and InSe/ZnSe films exhibited polycrystalline nature of crystallization. The optical analysis has shown that these devices exhibit a conduction band offsets of 0.47 and valence band offsets of 0.67 and 0.74[Formula: see text]eV, respectively. In addition, while the dielectric spectra of the InSe and ZnSe displayed resonance peaks at 416 and 528[Formula: see text]THz, the dielectric spectra of InSe/ZnSe and InSe/ZnSe/InSe layers indicated two additional peaks at 305 and 350[Formula: see text]THz, respectively. On the other hand, the optical conductivity analysis and modeling in the light of free carrier absorption theory reflected low values of drift mobilities associated with incident alternating electric fields at terahertz frequencies. The drift mobility of the charge carrier particles at femtoseconds scattering times increased as a result of the ZnSe sandwiching between two InSe layers. The valence band offsets, the dielectric resonance at 305 and 350[Formula: see text]THz and the optical conductivity values nominate TFT devices for use in optoelectronics.

The Influence of Diketopyrrolopyrrole Chemical Structure on Organic Field-Effect Transistors Performance

Organic semiconductors are suitable for application in biosensors and sensors based on transistors. The influence of soluble group modifications on the performance of diketopyrrolopyrrole-based organic field-effect transistors (OFETs) is studied. The lowest mobility 1·10-9 cm2/Vs was observed for non-symmetric substitution O,N. Measurable charge carrier mobility was observed due to reduction of the density charge trapping states after application of organosilane self-assembled monolayers (SAMs) on thinner gate-dielectrics (90 nm). We report similar drift mobility 1·10-7 cm2/Vs for smallest soluble group “butyl” as for biggest group “EthylAdamantyl” in N,N and O,O substitution prepared by spin-coating.

Effect of 100 kGy γ-irradiation on the structural, electrical and magnetic properties of CoFe2O4 NPs

Magnetic nanoparticles of CoFe2O4 NPs were fabricated using simple sol-gel auo-combustion synthesis and irradiated with total gamma dose of 50 kGy and 100 kGy maintaining a dose rate of 0.7 kGy/h. More emphasis was given for calculating an error free value of Lattice constant (α) by re-evaluating Debay-Sherrer value with Nelson-Riley extrapolation function (N-R). Bradley-jay function (B-J) and specimen mis-alignment error was elaborated while discussing the unit cell parameters. SEM images described the irradiation effects on polycrystalline and aggregated morphology of CoFe2O4 NPs. The prominent absorption bands (ν1, ν2, ν3, ν4) in room temperature FTIR spectra confirmed the formation of ferrite phase and stretching vibrations in γ-irradiated CoFe2O4 NPs. Decreasing DC-resistivity, increasing drift mobility and the measured range of activation energy (0.120–0.281 eV) was discussed as a function of γ-irradiation indicating ferrimagnetic behaviour of CoFe2O4 NPs. Several magnetic parameters (Mr, Hc, HK, nBe) were determined from the M–H loop recorded at room temperature. Magnetic anisotropy constant (K1) of gamma irradiated CoFe2O4 NPs was deduced using ‘Law of approach’ (LA) fit to saturation.

Association of structural and enhanced transport properties in RE substituted cobalt nanoferrites

Spinel ferrites belong to an important class of compounds which has variety of electrical and magnetic applications. Neodymium doped cobalt ferrites with composition CoFe2-xNdxO4 where x = 0.00, 0.05, 0.10, 0.15, 0.20 were synthesized by wet chemical route called simplified sol-gel method. Prepared samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) for structural analysis. All the samples were found spinel cubic belonging to Fd-3m space group. To analyze temperature effects on phase transition and to check thermal stability differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were done. Sample with x = 0.00 was found to be thermally more stable than remaining samples. Electrical analysis was done by using two probe method. DC electrical resistivity and drift mobility of sintered samples were measured as a function of temperature in the range from room temperature to 450 °C. It was observed that DC electrical resistivity has a direct dependence on dopant concentration but indirect dependence on temperature. Verwey hopping model was used to describe the results of DC electrical resistivity. Activation energy calculated from linear plots of DC resistivity for all samples was in the range of 0.532–0.594 eV. Rarely reported thermal transport properties that include thermal conductivity, thermal diffusivity and volumetric heat capacity were measured by Advantageous Transient Plane Source (ATPS) method. All the samples showed values near to that of thermal insulators. Thermal conductivity of all samples was increased up to 100–120 °C and then decreased. This behavior was thoroughly studied.

Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability.

On the basis of an excellent transistor material, [1]benzothieno[3,2-b][1]benzothiophene (BTBT), a series of highly conductive organic metals with the composition of (BTBT)2XF6 (X = P, As, Sb, and Ta) are prepared and the structural and physical properties are investigated. The room-temperature conductivity amounts to 4100 S cm(-1) in the AsF6 salt, corresponding to the drift mobility of 16 cm(2) V(-1) s(-1). Owing to the high conductivity, this salt shows a thermoelectric power factor of 55-88 μW K(-2) m(-1), which is a large value when this compound is regarded as an organic thermoelectric material. The thermoelectric power and the reflectance spectrum indicate a large bandwidth of 1.4 eV. These salts exhibit an abrupt resistivity jump under 200 K, which turns to an insulating state below 60 K. The paramagnetic spin susceptibility, and the Raman and the IR spectra suggest 4kF charge-density waves as an origin of the low-temperature insulating state.

Investigations of the drift mobility of carriers and density of states in nanocrystalline CdS thin films

Nanocrystalline Cadmium Sulfide (nc-CdS) thin films have been prepared on well-cleaned glass substrate at room temperature (300K) by thermal evaporation technique using inert gas condensation (IGC) method. X-ray diffraction (XRD) analysis reveals that the films crystallize in hexagonal structure with preferred orientation along [002] direction. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) studies reveal that grains are spherical in shape and uniformly distributed over the glass substrates. The optical band gap of the film is estimated from the transmittance spectra. Electrical parameters such as Hall coefficient, carrier type, carrier concentration, resistivity and mobility are determined using Hall measurements at 300K. Transit time and mobility are estimated from Time of Flight (TOF) transient photocurrent technique in gap cell configuration. The measured values of electron drift mobility from TOF and Hall measurements are of the same order. Constant Photocurrent Method in ac-mode (ac-CPM) is used to measure the absorption spectra in low absorption region. By applying derivative method, we have converted the measured absorption data into a density of states (DOS) distribution in the lower part of the energy gap. The value of Urbach energy, steepness parameter and density of defect states have been calculated from the absorption and DOS spectra.

Carrier transport property of truxene discotic liquid crystals with three different ring substituents

In this study, the charge carrier transport property of 3,8,13-trioctyloxytruxene [Trx(OC8)3] and its analogues, to which two different ring substituents of hydroxyl [Trx(OH)3(OC8)3] and methoxy [Trx(OMe)3(OC8)3] groups are introduced, has been studied relative to mesomorphism. Three analogues exhibit a hexagonal columnar (Colh) mesophase and their thermal stability increases with the introduction of hydroxyl and methoxy groups. The drift mobility measurements of Trx(OC8)3 and Trx(OH)3(OC8)3 reveal that the drift mobility is on the order of 5 × 10−2 cm2 V−1 s−1 in the Colh phase and it increases to 10−1 cm2 V−1 s−1 at the Colh-metastable phase transition, although Trx(OMe)3(OC8)3 shows a drift mobility of 1 × 10−2 cm2 V−1 s−1 in the Colh phase with temperature dependence. These results indicate that truxene with three alkoxy chains is an interesting molecular core for mesophase semiconductors.

Determination of deep trapping lifetime in organic semiconductors using impedance spectroscopy

A method for determining deep trapping lifetime in semiconductors using an impedance spectroscopy is proposed. A unique feature of the method is the simultaneous determination of the drift mobility and deep trapping lifetime in thin-film electronic devices. The validity of the proposed method is examined by numerical calculation. Simultaneous determinations of the drift mobility and deep trapping lifetime using this method are demonstrated in prototypical hole transporting organic semiconductors.

Isosbestic Point and Magnetoresistance Components in Ho $$_{0.5}$$ 0.5 Lu $$_{0.5}$$ 0.5 B $$_{12}$$ 12

We present the results of precise magnetoresistance (MR) measurements of an antiferromagnetic cage-glass solid solution Ho $$_{0.5}$$ Lu $$_{0.5}$$ B $$_{12}$$ in magnetic fields up to 80 kOe and at temperatures between 1.9 and 100 K. The analysis of results allows to separate the positive and negative contributions to MR. The dominant MR-term was found to be the high field positive contribution $$\Delta \rho /\rho _{(m+)} =\mu _\mathrm{D}^2 H^2$$ which is caused by the drift mobility $$\mu _\mathrm{D} $$ of charge carriers. It was also shown that the negative MR observed at temperatures $$T \le $$ 10 K may be described by the well-known relation between MR and local magnetization $$\Delta \rho /\rho _{(-)} =M_\mathrm{{loc}}^2 $$ obtained by Yosida in the framework of s–d exchange model. We discuss as well the scattering of charge carriers on antiferromagnetic nanosize domains of Ho $$^{3+}$$ ions with effective magnetic moments 3.5–5.5 $$\mu _\mathrm{B}$$ as the mechanism responsible for the appearance of the negative MR component.

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes.

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed.

Role of Ni–Co co-substitution in improving electrical and dielectric properties of La-based multiferroics nanomaterials

The synthesis of a material having ferroelectric and ferromagnetic properties in the same phase is a challenging task. In the present work, lanthanum based multiferroic materials, with composition La1−xCoxFe1−yNiyO3 (where, x=0.0–0.5 and y=0.0–1.0), have been synthesized via sol–gel auto-combustion method. The phase of the synthesized materials was confirmed by the X-ray diffraction (XRD) analysis, while the surface morphology and particle size were determined by the scanning electron microscopy (SEM) analysis. The DC electrical resistivity and activation energy were observed to increase from 2.14×107 to 3.04×1010Ωcm and 0.64 to 0.77eV, respectively with the increase in concentration of substituents. The drift mobility decreases from 3.27×10−13 to 2.80×10−16cm2V−1S−1. The dielectric constant, dielectric loss and dielectric loss factor decrease with the increase in frequency (1MHz to 3GHz) and Ni–Co content as well. The electrical and dielectric results are in good agreement with each other. The increase in electrical resistivity and decrease in dielectric parameters make these materials suitable for applications in microwave devices.

Optical interactions in the InSe/CdSe interface

In this work, the structural and optical properties of the InSe/CdSe heterojunction are investigated by means of X-ray diffraction and ultraviolet-visible light spectrophotometry techniques. The hexagonal CdSe films that were deposited onto amorphous InSe and onto glass substrates at a vacuum pressure of 10−5 mbar, exhibited interesting optical characteristics. Namely, the absorption, transmission, and reflection spectra that were recorded in the incident light wavelength range of 300–1100 nm, for the InSe, CdSe, and InSe/CdSe interface revealed direct allowed transition energy bandgaps of 1.44, 1.85, and 1.52 eV, respectively. The valence-band offset for the interface is found to be 0.36 eV. On the other hand, the dielectric constant spectral analysis displayed a large increase in the real part of the dielectric constant associated with decreasing frequency below 500 THz. In addition, the optical conductivity spectra that were analyzed and modeled in accordance with the Drude theory displayed a free-carrier average scattering time of 0.4 fs and a drift mobility of 6.65 cm2 V−1 s−1 for the InSe/CdSe interface. The features of this interface nominate it as a promising member for the production of optoelectronic Schottky channels and as thin-film transistors.

Synthesis and properties of bipolar derivatives of 1,3,5-triazine and carbazole

Three new bipolar star-shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine containing carbazolyl groups were designed, synthesized and characterized. All the materials possess high thermal stability and high glass transition temperatures ranging 97–226 °C. Photophysical study of the dilute solutions and neat films of the synthesized compounds was performed. Lippert–Mataga plots revealed linear dependence of Stokes shifts on the orientation polarizability for all the compounds. The dilute solutions of the triazine derivatives exhibited high photoluminescence quantum yields reaching 0.85, while for the neat films photoluminescence efficiency of 0.20–0.33 was observed. Ionization potentials of the solid layers of carbazole-triazine adducts estimated by photoelectron spectroscopy were found to be in the range of 5.49–5.97 eV. Hole drift mobility of the materials well exceeded the magnitude of 10−3 cm2 V−1 s−1 at an electric field of 6.4 · 105 V/cm. The selected compounds were tested as light emitting materials in organic light emitting diodes based on host-guest systems.

Mechanisms of temperature- and field-dependent effective drift mobilities and impact ionization coefficients in amorphous selenium

The mechanisms of electric-field- and temperature-dependent effective drift mobility and impact ionization coefficient of both holes and electrons in amorphous selenium (a-Se) are investigated in this paper. An analytical model for the microscopic mobility, momentum relaxation mean free path, and hence the effective drift mobility and impact ionization coefficient of carriers, is proposed in this paper by considering the density of states distribution, field enhancement release rate from the shallow traps, and carrier heating. The results of the model are fitted with the published experimental results on effective mobility and impact ionization coefficient with wide variations of the applied electric field and temperature. A better fitting considering thermally activated tunneling for the field-enhancement release rate indicates that the effective drift mobility at extremely high fields is mainly controlled by the neutral defect states near the band edges. The density of state function near the band edges, consisting of an exponential tail and a Gaussian peak, can successfully describe the electric-field- and temperature-dependent effective drift mobility characteristics in a-Se. The momentum relaxation mean free path decreases with increasing field and decreasing temperature, which is required to describe the electric-field- and temperature-dependent behaviors of impact ionization coefficient in a-Se.

Far tails of the density of states in amorphous organic semiconductors.

Far tails of the density of state (DOS) are calculated for the simple models of organic amorphous material, the model of dipolar glass and model of quadrupolar glass. It was found that in both models far tails are non-Gaussian. In the dipolar glass model, the DOS is symmetric around zero energy, while for the model of quadrupolar glass, the DOS is generally asymmetric and its asymmetry is directly related to the particular geometry of quadrupoles. Far tails of the DOS are relevant for the quasi-equilibrium transport of the charge carriers at low temperature. Asymmetry of DOS in quadrupolar glasses means a principal inequivalence of the random energy landscape for the transport of electrons and holes. Possible effect of the non-Gaussian shape of the far tails of the DOS on the temperature dependence of carrier drift mobility is discussed.

Monte-Carlo modeling of drift mobility in thin organic layers: lattice type influence

Hopping transport of charge carriers in thin (up to 100 nm) organic layers, which are suitable for organic light-emitting diodes and solar cells, is modeled in the framework of Gaussian disorder model. Drift mobility dependence within ranges of layer thickness, energy disorder and field strength is considered. Monte-Carlo simulations for low concentration case show weak dependence on type of lattice – simple cubic, body-centered and face-centered lattice. Results of this work are compared with the results obtained recently by Cottaar et al., done within master equation approach. The comparison shows the deviation at low disorder. Additionally we take into account influence of barrier-limited injection in case of cubic lattice.

Analysis and model of the photorefractive-like effect in non-conductive asymmetric liquid crystal hybrid cell

In this paper we describe a non-conductive liquid crystal hybrid cell with the photoconductive polymer (PP) layer. The orientation of the liquid crystal (LC) molecules in the described optical device is controlled by an external electric field and proper illumination. Method to calculate photoinduced refractive index distribution in liquid crystal layer has been presented. We propose model of the space charge formation in photoconductive polymer layer in the presence of light. We assumed that the carriers generated by the light in the PP diffuse into unlighted area and drift in external field and local field of the generated charge. In determining the distribution of the space charge field, we take into account the field-induced charges on the borders of the individual layers. After calculation of the final distribution of space charge and proper field distribution, we determine the director field distribution and refractive index distribution in LC layer. Full Text: PDF References A. Walczak, E. Nowinowski-Kruszelnicki, "Waveguide couplers induced optically over organic junction", Opt. Eng. 47(3), 035402 (2008). CrossRef A. Walczak, "Electromagnetic wave beam self - guidance in liquid crystal - semiconductor cell", Phot. Lett. Poland 2(2), 73 (2010). CrossRef A. Brignon, I. Bongrand, B. Loiseaux, J.P. Huignard, "Signal-beam amplification by two-wave mixing in a liquid-crystal light valve", Opt. Lett. 22(24), 1855 (1997). CrossRef G. Cook, C.A. Wyres, M.J. Deer, D.C. Jones, "Hybrid organic-inorganic photorefractives", Liquid Crystals VII, SPIE Annual Meeting (2003). CrossRef F. Kajzar, S. Bartkiewicz, A. Miniewicz, "Optical amplification with high gain in hybrid-polymer–liquid-crystal structures", Appl. Phys. Lett. 74(20), 2924 (1999) CrossRef S. Bartkiewicz, K. Matczyszyn, A. Miniewicz, F. Kajzar, "High gain of light in photoconducting polymer–nematic liquid crystal hybrid structures", Opt. Commun. 187, 257 (2001). CrossRef M. Kaczmarek, A. Dyadyusha, S. Slussarenko, I.C. Khoo, "The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers", J. Appl. Phys. 96, 2616 (2004). CrossRef P. Moszczyński, A. Walczak, P. Marciniak, "Model for simulation of photo-induced charges inside the hybrid LC cells", Phot. Lett. Poland 5(1), 11 (2013). CrossRef N. Kukhtarev, V. Markov, S. Odulov, M. Soskin, V. Vinetskii, "Holographic storage in electrooptic crystals. i. steady state", Ferroelectrics 22, 949 (1979). CrossRef N. Kukhtarev, T. Kukhtareva, "A unified treatment of radiation-induced photorefractive, thermal, and neutron transmutation gratings", IEEE Proceedings 87(11), 1857 (1999). CrossRef J.S. Schildkraut, Y. Cui, "Zero-order and first-order theory of the formation of space?charge gratings in photoconductive polymers", J. Appl. Phys. 72(11), 5055 (1992). CrossRef J.S. Schildkraut, A.V. Buettner, "Theory and simulation of the formation and erasure of space-charge gratings in photoconductive polymers", J. Appl. Phys. 72(5), 1888 (1992). CrossRef A. Twarowski, "Geminate recombination in photorefractive crystals", J. Appl. Phys. 65(7), 2833 (1989). CrossRef W.D. Gill, "Drift mobilities in amorphous charge-transfer complexes of trinitrofluorenone and poly-n-vinylcarbazole", J. Appl. Phys. 43(12), 5033 (1972). CrossRef A. Walczak, P. Moszczyński, E. Nowinowski-Kruszelnicki, "Semiconductor–LC Layer Boundary and Photonic Structures", Mol. Cryst. Liq. Cryst. 559(1), 186 (2012). CrossRef

Study of microstructure and augmentation of DC electrical resistivity due to Al3+ substitution in Ni–Zn nano ferrite system synthesized via auto combustion

Nanocrystalline Ni–Zn–Al spinel ferrite was synthesized via citrate-gel auto combustion method. The as-prepared powders have been separated into two batches in which one batch of powders were sintered at 1000[Formula: see text]C for 4 h and the other batch were pressed into pellets and were sintered at the same temperature. Sintering of the samples was done in air atmosphere followed by natural cooling to room temperature. The heat treated powders have then been characterized using TG–DTA, XRD, SEM and TEM for thermal, structural and microstructural aspects while the DC electrical resistivity measurements were carried out on the sintered pellets. The X-ray diffraction patterns displayed the formation of the spinel phase for all powders and the lattice parameter was obtained using Bragg’s law. The crystallite size for all compositions were found to be in nano dimensions and obtained from the Williamson–Hall method. TG–DTA analysis of the undoped [Formula: see text] indicated the formation of the spinel phase is around 400[Formula: see text]C while almost uniform microstructure with a more or less spherical grains has been noticed in the SEM micrograph. An enhancement in the DC electrical resistivity ([Formula: see text]-cm) has been observed in [Formula: see text] synthesized using this technique in comparison with that processed through conventional ceramic technique and a modification in the resistivity has been observed on substituting [Formula: see text] in place of [Formula: see text]. High electrical resistivity makes these ferrites suitable for high-frequency applications due to possible reduction of the eddy current losses. The observed variation in resistivity has been discussed on amendments in structure, microstructure and unavailability of [Formula: see text] ions with increasing [Formula: see text] ions in the light of existing understanding. The decrease in resistivity with increasing temperature confirms the semiconducting behavior of all samples. Activation energies for conduction were obtained from the slope of the log [Formula: see text] versus [Formula: see text] plots and observed to be in the range of 0.6–0.45 eV. The variation in the activation energy for conduction followed a similar trend as the DC resistivity. The drift mobility decreases with increasing [Formula: see text] ions concentration and increases with increasing temperature.

High-triplet-energy carbazole and fluorene tetrads

A series of twisted-structure carbazole–fluorene tetrads featuring 3,3′-bicarbazole core decorated with singly-bonded carbazole and fluorene end-linkers were synthesized and studied as potential host materials. The tetrads inferred high glass transition temperature (up to 105°C) and good film-forming properties, which contributed in attaining large hole drift mobilities (up to 10−3cm2/V/s) in the solution-processed amorphous films. The revealed high triplet energies (up to 2.9eV) indicated their potential to be utilized as hosts in blue light-emitting devices. Greater sensitivity of the triplet energies to the linking position of peripheral moieties as compared to the sensitivity of the singlets imply new possibilities for the independent tuning of these energies in carbazole–fluorene tetrads.