Field,temperature and thickness dependent electron transport in 5,5′-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-p-tolyl-1,3,4-oxadiazole)

Charge transport in 5,5′-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-p-tolyl-1,3,4-oxadiazole) is investigated as a function of temperature and organic layer thickness. The thickness dependence of the current indicates towards the trap charge limited conduction (TCLC) with a field and temperature dependent mobility. The density of trap states has been found to be dependent on sample thickness. As the thickness has increased from 80 nm to 120 nm, trap energy has correspondingly increased from 78 meV to 130 meV. TCLC model with Poole Frenkel type field dependent mobility has been fitted into the data and has been found in excellent agreement. Temperature dependency of zero field mobility (μ₀) and β has been estimated from the model.     

Electrical,optical and hole transport mechanism in thin films of poly(3-octylthiophene-co-3-hexylthiophene): Synthesis and characterization

The present study demonstrates the designing of copolymer poly(3-octylthiophene-co-3-hexylthiophene) (P3OT-HT) and study of the hole transport mechanism in it. Detailed structural, optical and thermal studies of P3OT-HT discuss its synthesis aspects. Current density–voltage characteristics have been studied at different temperatures (290–110 K) to understand the mechanism of hole transport in P3OT-HT. It has been established that current density in P3OT-HT thin films is governed by space charge limited conduction with traps distributed exponentially in energy and space. Hole mobility is both temperature and electric field dependent arising due to substituent functional groups attached the polymer backbone.     

Charge transport studies in thermally evaporated 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) thin film

Charge transport mechanism in 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenyl amino)-9,9′-spirobifluorene (spiro-MeOTAD) has been investigated as a function of temperature and organic layer thicknesses. Hole only devices of different thicknesses were fabricated in configuration ITO/spiro-MeOTAD/Au by vacuum evaporation technique. The hole current is space charge limited which provides a direct measurement of the hole mobility μ as a function of electric field and temperature. Gaussian disorder model has been used to explain the temperature and field dependent behavior of mobility. The values of energetic disorder (σ = 0.088 eV), positional disorder (Σ = 3.35) and mobility prefactor (μ₀ = 0.0147 cm²/V s) have been evaluated using this model.     

Charge carrier mobility in substituted polythiophene-based diodes

We have investigated the transport properties of the semiconducting polymer poly(3-(2′-methoxy-5′-octylphenyl)thiophene) (POMeOPT). We have measured the current–voltage (C–V) characteristics of single polymer layer devices in two regimes contact limited current and bulk-limited current. The passage from one regime to the other was done upon insertion of a conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS) between the metallic electrode and the semiconducting polymer. With PEDOT–PSS as electrode, the polymer gave space–charge limited current (SCLC) with the mobility dependent on electric field. Fitting the data, we were able to obtain important parameters, such as the zero-field mobility and the characteristic field. We have compared our results with the well-studied polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)–1,4-phenylene vinylene) (MEH–PPV) in similar experiments earlier reported.     

Spin dynamics control of recombination current in organic semiconductors

The recombination rate of coulombically bound electron–hole pairs depends on their spin configuration. Because of low spin–orbit coupling the spin dynamics of these well separated coulombic pairs is determined by weak hyperfine and exchange interactions. In this case a weak magnetic field produces strong effect on the spin dynamics and hence on the recombination rate of e–h pairs. We have shown that the recombination current in organic semiconductors may have a maximum as a function of recombination constant. For high recombination constants the current is space charge limited and decreases with increasing the e–h recombination constant. This decay of current is due to decrease of the region where the recombination takes place. At a low recombination constant the recombination takes place in the whole volume and the current increases with increasing the recombination constant. The characteristic recombination constant separating those two regimes depends on the thickness of sample, applied voltage, and charge carrier mobilities. The model predictions are consistent with experimental data for magnetoresistance of organic semiconductors.     

氧化锆基固体电解质离子导电的基本原理

立方ZrO2具有高的离子传导性能,近年来被广泛用作固体氧化物燃料电池的电解质。固体中离子电导率决定于载体浓度（密度）和迁移率,而固态离子的传导和扩散则决定于固体中的缺陷结构。ZrO2的传导性能与掺杂剂的组成、晶体结构、晶粒大小和温度有关。文中着重论述了作为电解质的稳定立方ZrO2中O2^-离子传导的作用原理和掺杂机制。     

Electrical and impedance spectral characterisation of ITO/DAG/In device

A Schottky device, with configuration ITO/DAG/In is fabricated using diazopheny diamino glyoxime (DAG) as an n-type organic material. Current–voltage characteristics and impedance spectroscopy measurements were carried out, which reveals that the injection and transport properties are dominated by negative charge carriers. Space charge limited current theory with an exponential distribution of traps is very well followed by observations resulted through current–voltage characteristics at high voltage region. This gives a traps density N_t of about 4.5×10²¹ m⁻³ and mobility of electron is about 3.9×10⁻¹⁰ m² V⁻¹ s⁻¹. It is found that DAG behaves as an n-type materials as it forms Schottky barrier with ITO (high work function electrode) and conduction is governed by majority carriers, i.e. electrons. Using temperature and bias dependence of impedance spectral characteristics in a broad frequency range, i.e. 40 Hz to 100 kHz, it is found that the ac behaviour of In/DAG/ITO device shows several features, described by the simple double RC circuit representing a depleted junction region and an undepleted bulk region. From the large frequency range of impedance spectroscopy two distinct processes were identified, corresponding to bulk DAG layer and junction region. The activation energy of the relaxation times coincides well with the results obtained from the temperature dependent dc conductivity. The temperature dependent capacitance–voltage measurements were analysed at a frequency of 40 Hz. The obtained inherent donor concentration from 1/C²–V plots varies from 1.8×10²³ m⁻³ at room temperature to 5.9×10²³ m⁻³ at 360 K.     

Charge conduction process and photoelectrical properties of Schottky barrier device based on sulphonated nickel phthalocyanine

In this paper, the optical and electrochemical properties of sulphonated nickel phthalocyanine (NiPcS_mix) were investigated. The ground state of spectra of NiPcS_mix show splitting of Q band in DMF, but the fluorescence spectra have only one band, suggesting that only some component of the sulphonated NiPc fluoresce. Since, the organic materials are described on the basis of molecular orbital energies, i.e. highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) instead of valence band and conduction band. Such energies were estimated from cyclic voltammetry and optical absorption measurements. The dark current–voltage characteristics of ITO/NiPcS_mix/Ag device in dark at room temperature shows a rectification behavior. At low voltages, current in forward direction was found to obey the diode equation, i.e. exponential increase in current with applied voltage and the conduction was controlled by thermionic emission mechanism. For relatively higher voltages, the conduction was dominated by a space charge limited conduction mechanism with single trap level of 0.34 eV. The J–V characteristics in dark and the comparison of photoaction spectra of the device with the optical absorption spectra of NiPcS_mix indicates the formation of Schottky barrier and Ohmic contact at Ag-NiPcS_mix and ITO-NiPcS_mix, respectively, in ITO/NiPcS_mix/Ag device. The junction parameters such as built in potential, potential barrier height, carrier concentration and width of depletion layer were estimated from the capacitance–voltage (C–V) measurement. The J–V characteristic under illumination shows photovoltaic effect. The power conversion efficiency of the device has been improved upon thermal annealing attributed to the enhancement in the both crystallinity of NiPcS_mix and charge carrier mobility due to the thermal treatment. We have also investigated the effect of PEDOT:PSS buffer layer in between ITO and NiPcS_mix on electrical and photovoltaic properties of the device.     

Modeling of mobility in organic thin-film transistor based octithiophene (8T)

Thin-film organic field-effect transistor was made with vapor-deposited polycrystalline octithiophene on silicon oxide insulator layers. In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductors/insulator interface in to the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface. In this paper we present a new approach to the mobility in organic thin-film transistor (OTFTs), and with a new procedure we extract the electrical parameters of organic TFTs that possible to reproduce very well the device characteristic and mobility.     

Anomalous room temperature magnetoresistance in organic semiconductors

We report the substantial change in the large room temperature (∼8% at 100 Oe, up to 15% at 1000 Oe) magnetoresistance of thin organic semiconductor films of tris-(8-hydroxyquinoline) aluminum (Alq₃) upon doping with PtOEP and Ir(ppy)₃ complexes. The origin of magnetic field effects on charge transport properties of organic semiconductors until now has remained obscure. We propose a model for the anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron–hole (e–h) recombination limited. The process of e–h recombination includes formation of correlated e–h pairs and the subsequent annihilation of e–h pairs with different rates for the singlet and triplet spin states. The e–h pairs may also dissociate back into free charge carriers. We suggest that a magnetic field controls spin interconversion of e–h pairs. In the absence of field the singlet mixes with the entire triplet manifold by hyperfine interaction. The magnetic field lifts the triplet degeneracy, and for strong field, the mixing remains only between the singlet and the T₀ component of the triplet, thus changing the e–h recombination rate and hence the current. The experimental results are consistent with the model.     

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.     

Studies on electrical and photoelectrical behaviour of ITO/ArV/In Schottky barrier device

The electrical and photoelectrical properties of aryl viologen (ArV), chemically known as 1,1′-diphenyl-4,4′-bipyridinium dichloride, in the form of thin film, sandwiched between ITO and In electrode were studied. The current–voltage (J–V) characteristics in dark show the rectification effect due to the formation of Schottky barrier at In–ArV interface. The diode quality factor of the device, greater than unity, indicates the recombination of electron-hole in depletion region. Ohmic conduction in low voltage range and space charge limited conduction (SCLC) controlled by an exponential distribution of traps above the valence band edge, for higher voltage region have been observed. Various electrical parameters were calculated from the analysis of J–V and capacitance–voltage (C–V) characteristics at different temperatures and discussed in details. At higher frequencies, the device exhibit voltage independent capacitance, which is explained in terms of the extremely slow kinetics of space charge and low mobility of charge carriers. The photoaction spectra of the device and absorption spectra of the ArV thin film reveal that the fraction of light, which is absorbed near or within the diffusion length of exciton, is responsible for producing the free charge carriers. The photovoltaic parameters were calculated from the J–V characteristics under illumination through ITO and discussed in detail.     

On the field and temperature dependence of hole mobility in molecularly doped polymer

The charge transport properties of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped polycarbonate (PC) have been measured as a function of field and temperature. Hole mobility is observed to follow log μ ∝ E^1/2 (i.e. Poole–Frenkel behavior) at relatively high field strengths while at low field strengths mobility initially decreased with increase in field strength. Mobility value undergoes minima at a particular field value for a given temperature. Minima in mobility are observed to occur at higher field strength as the temperature is lowered. Mobility data and transport parameters are analyzed using the formalism of Gaussian disorder model (GDM). The mobility behavior at low field and shift of mobility minima with temperature suggests large disorder in the film. Investigation of film morphology using photoluminescence, X-ray diffraction and scanning electron microscopy suggests that large positional disorder in the film is probably due to film morphology and TPD–PC interaction.     

Comparison of the carrier mobility,unipolar conduction,and light emitting characteristics of phosphorescent host–dopant system

We have investigated the charge carrier transport mobility and hole-only current behavior of phosphorescent host–dopant mixture, and evaluated the efficiency and lifetime behavior of organic light emitting device (OLED) containing the corresponding light emitting host–dopant system. The carrier drift mobilities of the phosphorescent host, 4,4′-N,N′-dicarbazole-biphenyl (CBP), doped with green-emitting fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) or bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetyl-acetonate) (Btp₂Ir(acac)) red-emitting dopants, were directly investigated with time of flight (TOF) photoconductivity method. The resolved electron mobility of phosphorescent host–dopant layer by TOF-PC method showed the significant reduction at CBP:(Btp₂Ir(acac)) layer, which can be explained by the electron trapping. Measured hole-only current data also shows the reduction, which is more significant at CBP:(Btp₂Ir(acac)) as expected from larger energy level offset. The efficiency, spectral emitting properties, and device stability of phosphorescent OLEDs with identical host–dopant layer were evaluated. Compared with CBP:(Btp₂Ir(acac)), device with CBP:Ir(ppy)₃ emitter shows the spectral response and half-lifetime less dependent upon the hole/exciton blocking layer and its thickness. Such device data was well-correlated with probed TOF and hole current behavior.     

Space charge limited current–voltage characteristics of organic semiconductor diode fabricated at various gravity conditions

In this paper, temperature-dependent current–voltage (I–V) characteristics of poly-N-epoxipropyl carbazole (PEPC) are evaluated. The PEPC is doped with anthracene (An) and deposited on nickel (Ni) substrate with a centrifugal machine. The films are grown at room temperature but at varying gravity conditions, such as 1g, 123g, 277g and 1107g, where g is acceleration due to gravity. It is demonstrated that the space charge created by the trapped charges controls the device's characteristics. Thus, by employing trapped space charge limited current model, charge transport parameters are estimated and discussed as a function of ambient temperatures. It is learned that the trap factor, free carrier density, effective mobility and trap density are quasi-linear functions of temperatures. It is shown that devices fabricated at 277g exhibit superior electrical properties compared to 1g, 123g and 1107g devices. It has been demonstrated that an organic semiconductor device performance could be enhanced by optimizing its fabrication parameters.     

Tunneling vs. giant magnetoresistance in organic spin valve

We studied magnetoresistance (MR) in La_2/3Sr_1/3MnO₃ (LSMO)/organic semiconductor (OSC)/Fe heterojunction devices using rubrene (C₄₂H₂₈) as an organic semiconductor. Efficient spin polarized tunneling using a hybrid barrier (oxide (1.2 nm)/rubrene (5 nm)) was observed. Devices with a thin layer of rubrene as the barrier may have magnetic clusters and/or pinholes in the barrier, which could explain significant variations of MR among devices. As the thickness of the rubrene layer is increased, device current becomes strongly limited by carrier injection resulting in strong temperature and bias dependent device resistance. The carrier injection in these devices can be described with thermionic field emission at the metal/OSC interface and is analyzed with both empirical and theoretical models. The effect of carrier transport through the spacer on the magnetoresistance for organic-based spin valve is discussed. The observed giant magnetoresistance (GMR) in 20 nm rubrene device demonstrates the spin polarized carrier injection and transport through the rubrene OSC layer.     

Double-walled carbon nanotube/polymer nanocomposites: Electrical properties under dc and ac fields

The electrical properties of the poly(methyl methacrylate:carbon nanotubes nanocomposites) have been investigated by direct current conductivity and complex impedance spectroscopy methods. The direct current conductivity results of the poly(methyl methacrylate):carbon nanotube as a nanocomposites show that the electrical conductivity property of the poly(methyl methacrylate) changes from insulating state to semiconducting state with incorporation of double wall carbon nanotube DWCNTs into insulating polymer matrix. The alternating current conductivity mechanism of the nanocomposites is controlled by the correlated barrier hopping mechanism. The correlated barrier hopping CBH model for intimate valence alternation pairs IVAP's describes the conduction mechanism of PMMA doped with (1%) DWCNTs, while correlated barrier hopping CBH model for non-intimate valence alternation pairs describes the conduction mechanism of PMMA doped with (5% and 8%) DWCNTs. The real part of the complex impedance decreases with the increase of the applied frequency which revealed that the PMMA:DWCNT nanocomposites behaves like semiconducting materials. The complex impedance Nyquist plots for PMMA doped with different concentration DWCNTs over are characterized by the appearance of a single semicircular arc whose radii of curvature decreases with increasing the temperature. Cole and Cole plots show the presence of temperature dependent electrical relaxation phenomena in the PMMA:DWCNT nanocomposites. The obtained electronic parameters confirm that PMMA:DWCNTs exhibit organic semiconductor behavior.     

Carrier concentration dependence of the mobility in organic semiconductors

Charge transport in organic materials as a function of carrier concentration is investigated. An analytical model of the concentration dependent mobility based on the variable hopping range theory is formulated. This model is applied to analyze the discrepancy between the experimental mobilities extracted from FETs and LEDs. The result shows that an exponential density of states (DOS) is a good approximation of the tail states for describing the charge transport in FETs. When applied to the low carrier concentration regime, for example to the LEDs regime, a Gaussian DOS should be assumed.