Drift Mobility Of Charge Carriers Research Articles

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency.

The general fabrication process for stabilized amorphous selenium (a-Se) detectors is vacuum deposition. The evaporant alloy is typically selenium alloyed with 0.3–0.5%As to stabilize it against crystallization. During the evaporation, fractionation leads to the formation of a deposited film that is rich in As near the surface and rich in Se near the substrate. The As content is invariably not uniform across the film thickness. This paper examines the effect of non-uniform As content on the charge collection efficiency (CE). The model for the actual CE calculation is based on the generalized CE equation under small signals; it involves the integration of the reciprocal range-field product (the schubweg) and the photogeneration profile. The data for the model input were extracted from the literature on the dependence of charge carrier drift mobilities and lifetimes on the As content in a-Se1−xAsx alloys to generate the spatial variation of hole and electron ranges across the photoconductor film. This range variation is then used to calculate the actual CE in the integral equation as a function of the applied field. The carrier ranges corresponding to the average composition in the film are also used in the standard CE equation under uniform ranges to examine whether one can simply use the average As content to calculate the CE. The standard equation is also used with ranges from the spatial average and average inverse. Errors are then compared and quantified from the use of various averages. The particular choice for averaging depends on the polarity of the radiation-receiving electrode and the spatial variation of the carrier ranges.

Band offsets, dielectric dispersion and some applications of CdSe/GeO2 heterojunctions

Herein, the formation, structural, morphological, compositional, optical, dielectric, photoelectrical and electrical properties of the CdSe/GeO2 heterojunctions are explored. While the surface displayed formation of deficient GeO, the bulk of the films exhibited correct stoichiometry of GeO2 and CdSe. It is found that stacking of GeO2 onto CdSe enhances the crystallinity of CdSe by reducing the defects concentrations. It blue shifts the energy band gap of CdSe from 1.74 eV to 2.60 eV. The stacked layers of CdSe/GeO2 displayed conduction and valence band offsets of 2.34 eV and 1.23 eV, respectively. In addition, the Drude-Lorentz model analyses have shown that the coating of GeO2 onto CdSe, highly increases the drift mobility of charge carriers and tunes the plasmon frequency making it more appropriate for optoelectronic device applications. When exposed to light irradiation, an illumination intensity dependent photosensitivity is observed. The photoconduction mechanism is governed by the sublinear recombination with exponential trap distribution. Moreover, the capacitance –voltage characteristics of the device revealed metal-oxide-semiconductor field effect transistors (MOSFET) characteristics. The built in potential for this device under reverse biasing conditions reached 4.74 eV.

Polaron conduction mechanism in Nickel ferrite and its rare-earth derivatives

NiFe2O4 & NiFe1.925R0.075O4 (R = Lu, Yb & Y) samples conductivity response for alternating current (σac) in the high temperature region (300 K to 570 K), and for direct current conductivity (σdc) response in the temperature range 80 K – 300 K were investigated. For all the investigated samples σac increases with raising frequency owing to transfer of charge carriers at the two octahedral sites containing Fe3＋ or Ni2+. The total σac was observed to follow σ = D + Bωn where the frequency exponent n, initially decreases and then increases following overlapping large polaron model. The behaviour of σac with temperature reveals the semiconducting behaviour of the investigated samples. The Arrhenius plots of σac shows two regions of conductivity suggesting two types of relaxation phenomena. The σdc, also increases with increasing temperature, owing to the raise in the thermally stimulated drift mobility of charge carriers. The decrease in the activation energy with decreasing temperature corresponds to small polaron hopping mechanism.

An alternative method for measurement of charge carrier mobility in semiconductors using photocurrent transient response

We report here a simple alternative method for measuring charge carrier drift mobilities in semiconductor devices. A typical falling photocurrent transient formula for switch-off-state was adjusted to obtain simultaneously electron and hole mobilities. For both undoped ZnO film and InAlAs/InGaAs quantum well structure, electron mobilities extracted from our model were compared with those obtained from maximum-entropy mobility-spectrum analysis method (ME-MSA). Our results demonstrated that electron mobility obtained from our photocurrent response model could serve as substitutes for a representative mobility obtained from ME-MSA.

Synthesis and investigation of charge transport properties in adducts of hole transporting carbazole derivatives and push-pull azobenzenes

In order to investigate the viability of a material design for bulk heterojunction (BHJ) organic solar cells, where hole transporting group is bound to the donor moiety, we report the synthesis and charge transport characteristics of 3-(diphenylamino)carbazolyl-functionalized derivatives of 2-(4-((4-(dimethylamino)phenyl)diazenyl)benzylidene)-1H-indene-1,3-dione (DMAAzi) chromophore. Three different bounding configurations were examined in these adducts. Additionally, a trityl-functionalized derivative of DMAAzi was prepared and used for comparison purposes. All of the synthesized materials form thin amorphous films from volatile organic solvents and exhibit glass transition temperatures in the range from 89 °C to 124 °C. The molecular ionization energy and electron affinity energy levels in thin films were measured. Photo-induced time of flight (ToF) method was used in to determine charge carrier drift mobilities. It was found out that the formation of deep charge trap states with local energies at approximately 0.60–0.78 eV takes place and has a considerable negative effect on the hole drift mobility of the investigated compounds.

Design and characterization of MoO3/CdSe heterojunctions

In this work, the morphological, compositional, structural, optical and dielectric properties of CdSe which are deposited onto glass and onto MoO3 thin film substrates are investigated. The use of MoO3 as substrate for the growth of CdSe is observed to increase the lattice parameters of the hexagonal unit cell of CdSe and decreases the values of grain size and strain. It also forms band tails of width of 0.20 eV in the band gap of CdSe. The optical analysis has shown that the MoO3/CdSe interfacing results in blue shift in the energy band gap of CdSe and also result in large conduction and valence band of sets of values of 2.12 and 0.94 eV, respectively. The dielectric spectral analysis with the help of Drude-Lorentz approaches for optical conduction, revealed an enhancement in the drift mobility of charge carriers from 15.69 to 39.30 cm2/V as a response to the incident electromagnetic field. The free carrier density of the MoO3/CdSe being of the order of 1017 cm−3 with the large valence and conduction band offsets and the sufficiently large drift mobility nominates the MoO3/CdSe heterojunctions as an effective component of optoelectronic technology including thin film transistors.

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.

Design of Janus triphenylene mesogens: Facile synthesis, mesomorphism, photoluminescence, and semiconductivity

A new variety of non-symmetrical triphenylene-based discogens has been synthesized by intramolecular Scholl cyclodehydrogenation reaction of kinked terphenylenes using the FeCl3/MeNO2 couple as an efficient oxidant. The “Janus-like” tetrakis(alkoxy)triphenylene discogens containing electron-withdrawing groups, such as cyano-, chloro- and fluoro-substituents, or electron-donating moieties, such as carbazole, thiophene and naphthalene at the opposite of the alkylated side were prepared in good to high yields. Polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) investigations revealed that six of the 14 discogens synthesized exhibit a hexagonal columnar mesophase. These chromophores emit bright blue light from 350 nm to 500 nm in solution depending on the extension of their π-conjugation. In addition, all six columnar liquid crystals show non-dispersive time-of-flight photocurrent transportation, with charge carrier drift mobility in the range of 10−3 to 10−4 cm2 V−1 s−1, higher or in the same range as their symmetrical hexa(alkoxy)triphenylene parent compounds.

Synthesis of Bi3+ substituted Ni-Cu ferrites and study of structural, electrical and magnetic properties

Bi3+ substituted Ni-Cu ferrites with the particles size 0.28 μm–0.53 μm were prepared by solid state reaction method. X-ray diffraction analysis confirmed the single phase cubic spinel structure with the space group of Fd3m-Oh7. IR absorption bands assigned at 580 cm−1 and 403 cm−1 corresponds to the stretching of vibrations of tetrahedral and octahedral complexes of the ferrites. The average grain sizes of the ferrites were estimated by SEM micrographs at around 1.04 μm which was found to be increased with Bi content. The decrease of DC resistivity as a function of temperature shows the semiconducting nature of ferrites and is attributed to the increase in thermally activated drift mobility of charge carriers. The dielectric constant decreases with increase in the frequency of all samples shows dispersion behavior. Hysteresis loop of the ferrites at room temperature were studied for the determination of saturation magnetization and magnetic moment. The maximum saturation magnetization obtained is about 41.27emu/gm for Ni0.5Cu0.5Fe2O4 ferrite.

Thermal stability and photoconductive properties of photosensors with an alternating multilayer structure of amorphous Se and AsxSe1−x

In this study, we fabricated a-Se based photosensors with an alternating multilayer structure of a-Se and AsxSe1−x by rotational thermal evaporation deposition. During the deposition of the amorphous AsxSe1−x layers, As diffuses into the underlying a-Se component layers, thereby improving the thermal stability of the multilayer photosensor and thus increasing the breakdown electric field. Although the As doping introduces carrier traps in the a-Se layers, the multilayer photosensors demonstrate an effective quantum efficiency comparable to the single-layered a-Se sensor under the blue light illumination but are with a lower dark current density by two orders of magnitude. In addition to the top AsxSe1−x layer being functioning as an electron blocking layer, carrier traps present in the multilayer structure may decrease the drift mobility of charge carriers and disturb electric field distribution in the photosensors, thereby suppressing the dark current.

Distribution of charge carrier transport properties in organic semiconductors with Gaussian disorder

The charge carrier drift mobility in disordered semiconductors is commonly graphically extracted from time-of-flight (TOF) photocurrent transients yielding a single transit time. However, the term transit time is ambiguously defined and fails to deliver a mobility in terms of a statistical average. Here, we introduce an advanced computational procedure to evaluate TOF transients, which allows to extract the whole distribution of transit times and mobilities from the photocurrent transient, instead of a single value. This method, extending the work of Scott et al. (Phys. Rev. B 46, 8603 (1992)), is applicable to disordered systems with a Gaussian density of states and its accuracy is validated using one-dimensional Monte Carlo simulations. We demonstrate the superiority of this new approach by comparing it to the common geometrical analysis of hole TOF transients measured on poly(3-hexyl thiophene-2,5-diyl). The extracted distributions provide access to a very detailed and accurate analysis of the charge carrier transport. For instance, not only the mobility given by the mean transit time but also the mean mobility can be calculated. Whereas the latter determines the macroscopic photocurrent, the former is relevant for an accurate determination of the energetic disorder parameter σ within the Gaussian disorder model. σ derived by using the common geometrical method is, as we show, underestimated instead.

Photophysical properties of 2-phenylanthracene and its conformationally-stabilized derivatives

A synthetic route to a series of anthracene derivatives consisting of 2-phenylanthracene and its conformationally-stabilized planar derivatives, containing carbon, oxygen, sulphur and nitrogen atoms, is reported. Thermal, optical and electrical properties of the derivatives featuring an extended π-conjugated electron system were also investigated. Experimental results were supplemented by the energy spectra and molecular orbital calculations performed in the framework of density functional theory. Some of the compounds were found to be molecular glasses with glass transition temperatures from 28 to 55 °C as revealed by differential scanning calorimetry. The anthracene compounds in dilute solutions were found to emit in the 400–500 nm wavelength range with the fluorescence quantum yields varying from 0.22 up to 0.46. The solid films of the anthracene derivatives exhibited slightly narrower emission wavelength-tunability with clearly-expressed excimer emission bands (for the benzothiophene- and indole-anthracene compounds) causing a significant lowering of the fluorescence quantum efficiency down to 0.005. Rigidity of the studied anthracene molecules was assessed by evaluating the Huang-Rhys factor (indicating strength of electron-vibrational coupling) from the vibronic profile of the fluorescence spectra, which indicated that the cyclized indole- and benzofuran-anthracenes possess the most rigid molecular structures as opposed to more flexible structures of 2-phenylanthracene and indeno-anthracene. Wet-casted films of the anthracene compounds demonstrated moderate charge carrier drift mobilities, in the order of 10−5–10−4 cm2/V, which are considered as sufficient for light-emitter applications. Moreover, introduction of additional C, O, S and N atoms into 2-phanylanthracene molecule enabled a tuning of the HOMO level from 4.9 up to 5.74 eV, which is crucial for the optimization of hole injection into the active layers, and thus for balancing electron and hole currents in light-emitting devices.

Drift Mobility, Diffusion Coefficient of Randomly Moving Charge Carriers in Metals and Other Materials with Degenerated Electron Gas

In this short review some aspects of applications of free electron theory on the ground of the Fermi statistics will be analyzed. There it is an intention to attempt somebody’s attention to problems in widespread literature of interpretation of conductivity of metals, superconductor in the normal state and semiconductors with degenerated electron gas. In literature there are many cases when to these materials the classical statistics is applied. It is well known that the electron heat capacity and thermal noise (and as a consequence the electrical conductivity) are determined by randomly moving electrons, which energy is close to the Fermi energy level, and the other part of electrons, which energy is well below the Fermi level can not be scattered and change its energy. Therefore there was tried as simple as possible on the ground of Fermi distribution, and on random motion of charge carriers, and on the well known experimental results to take general expressions for various kinetic parameters which are applicable for materials both without and with degenerated electron gas. It is shown, that drift mobility of randomly moving charge carriers, depending on the degree degeneracy, can considerably exceed the Hall mobility. Also it is shown that the Einstein relation between the diffusion coefficient and the drift mobility of charge carriers is valid even in the case of degeneracy. There also will be presented the main kinetic parameter values for different metals.

Dielectric behavior and AC conductivity of Tb3+ doped Ni0.4Zn0.6Fe2O4 nanoparticles

Terbium doped Ni–Zn ferrite, Ni0.4Zn0.6TbxFe2−xO4 (x=0, 0.05, 0.1, 0.15 and 0.2) have been prepared through sol–gel technique. Formation of single phase FCC spinel structure is identified in all the compositions using X-ray diffraction technique. The effect of Tb3+ doping on the crystalline phase, crystallite size, dielectric properties and AC conductivity has been investigated in detail. Lattice constant is observed to increase with terbium content; but beyond certain concentration of terbium (x>0.15), it decreased. DC resistivity measurements were carried out in the temperature range 300–523K. All the compositions exhibited usual semiconducting nature of ferrites and the resistivity of the samples found to increase with terbium content up to x=0.15. The metallic behavior of the doped samples observed in the lower temperature region is explained in terms of elastic and inelastic scattering due to the rare earth ions. The investigations on dielectric constant, dielectric loss and AC conductivity was carried out in the frequency range 100Hz–20MHz from room temperature to 563K. Frequency dependence of dielectric constant showed normal behavior and is in good agreement with Koop’s phenomenological theory of dielectric dispersion. AC conductivity measurements suggested conduction due to small polaron hopping. AC conductivity as well as dielectric constant is found to increase with temperature and which is due to thermally enhanced drift mobility of charge carriers. The observed decrease in dielectric loss with terbium content up to x=0.15 is explained in terms of increase in resistivity.

Effect of excessive pressure on the drift mobility of charge carriers in poly(diphenylene phthalide) films

The electron-hole transport in poly(diphenylene phthalide) films has been investigated. The dependence of the drift mobility of charge carriers on the excessive mechanical pressure has been studied using the time-of-flight method. It has been revealed that, with an increase in the thickness of the polymer film, the dispersive transport of charge carries gives way to the quasi-dispersive transport. In thin films in the prethreshold range (i.e., before switching of the samples to the highly conductive state under excessive pressure), the electron mobility increases and exceeds the hole mobility. The experimental results have been discussed in the framework of the model describing the transport through the channels formed by metastable electron-hole pairs.

Charge carrier mobility in poly[methyl(phenyl)silylene] studied by time-resolved terahertz spectroscopy and molecular modelling

Time-resolved terahertz spectroscopy and combination of quantum chemistry modeling and molecular dynamics simulations were used for the determination of charge carrier mobility in poly[methyl(phenyl)silylene]. Using time-resolved THz spectroscopy we established the on-chain charge carrier drift mobility in PMPSi as 0.02 cm(2) V(-1) s(-1). This value is low due to the formation of polarons: the hole is self-trapped in a potential formed by local chain distortion and the transient THz conductivity spectra show signatures of its oscillations within this potential well. This view is supported by the agreement between experimental and calculated values of the on-chain charge carrier mobility.

Transient electroluminescence and anomalous dispersion of charge carriers in thin polymer films

The electroluminescence and charge carrier transport in an organic light-emitting diode based on a thin polymer film are investigated. The initial kinetics of transient electroluminescence is analyzed theoretically. Both the theoretical and experimental curves are characterized by an anomalous dispersion and universality, which can be considered a consequence of the nonequilibrium field-assisted dispersion of charge carriers. It is demonstrated that the drift mobility of charge carriers should be determined from the time corresponding to the half-height of the steady-state electroluminescence intensity rather than from the delay time of the electroluminescence.

High performance organic photovoltaic devices using amorphous molecular materials with high charge-carrier drift mobilities

p n -Heterojunction organic photovoltaic devices (OPVs) using amorphous molecular materials with high hole drift mobilities of 10−2 cm2 V−1 s−1, tris[4-(2-thienyl)phenyl]amine and tris[4-(5-phenylthiophen-2-yl)phenyl]amine, as electron donors and fullerene as an electron acceptor were fabricated. In spite of the use of amorphous materials instead of crystalline materials, the devices exhibited high performance with fill factors of 0.62–0.71 and power conversion efficiencies of 1.5%–1.7% under air-mass 1.5 G illumination at an intensity of 100 mW cm−2.

Influence of ambient atmosphere on charge transport in polycrystalline thin films of three simple aromatic hydrocarbons

Drift mobility of charge carriers in p-terphenyl, p-quaterphenyl and tetracene polycrystalline thin films was measured using the classical time-of-flight method. The measurements were carried out both in the atmosphere and in vacuum of the order of 10( - 3) Torr. The mobility measured in the atmosphere is of the order of 10(-4)cm2 V(-1) s(-1) and turns out to be about 1 order of magnitude higher than that measured in vacuum. It is suggested that the hopping transport of charge dominates in the structures investigated. The increase in the mobility for the measurements in the atmosphere probably results from the additional localized states due to water and other atmosphere molecules.

Relation between physical structure and electrical properties of diamond-like carbon thin films

Diamond-like carbon (DLC) belongs to very interesting materials used for a number of practical applications. Its properties strongly depend on the method of deposition and on the deposition parameters. In particular, the electrical properties of DLC films obtained by RF PCVD discharge depend substantially on the preparation conditions. Influence of deposition conditions on the physical structure and chemical content is discussed. Both the drift mobility of charge carriers and the electrical conductivity of DLC films strongly depend on deposition conditions. The electrical conductivity results are explained in terms of hopping mechanism. At the temperatures below 260 K, the hopping among states in a narrow band of states dominates; at the higher temperatures, the band-to-band transitions are suggested to occur. A model of the forbidden gap explaining the observed electrical properties of DLC films is proposed. Experimental results concerning the electrical conduction of DLC/Si heterostructures are also presented and discussed. The difference in the properties of DLC/n-Si and DLC/p-Si structures may suggest that the electron transport via localised states dominate in the investigated DLC films. Recombination lifetime of charge carriers in DLC films is measured using both dielectric spectroscopy and detection of decay of photocurrents. The lifetime is found to be of the order of 0.3–0.4 ms, which is quite long in comparison with classical doped silicon semiconductors.