Characterization of the hole transport and electrical properties in poly(9,9-dioctylfluorene)

A systematic study of the hole transport and electrical properties in blue-emitting polymers as poly(9,9-dioctylfluorene) (PFO) has been performed. We show that the temperature dependent and thickness dependent current density versus voltage characteristics of PFO hole-only devices can be accurately described using our recently introduced improved mobility model based on both the Arrhenius temperature dependence and non-Arrhenius temperature dependence. Within the improved model, the mobility depends on three important physical quantities: temperature, carrier density, and electric field. For the polymer studied, we find the width of the density of states σ=0.115 eV and the lattice constant a=1.2 nm. Furthermore, we show that the boundary carrier density has an important effect on the current density versus voltage characteristics. Too large or too small values of the boundary carrier density lead to incorrect current density versus voltage characteristics. The numerically calculated carrier density is a decreasing function of distance from the interface. The numerically calculated electric field is an increasing function of distance. Both the maximum of carrier density and minimum of electric field appear near the interface.     

Molecular geometry fluctuation model for the mobility of conjugated polymers

We present a model to describe electrical transport in dense films of conjugated polymers. The essential physical features of the model are as follows: (i) thermal fluctuations in the molecular geometry of the polymer modify the energy levels of localized electronic states in the material, and (ii) the primary restoring force for these fluctuations is steric, which leads to spatial correlation in the energies of the localized electronic states. The model describes the electric field dependence of the mobility and explains the carrier density dependences of mobility observed in polymer diodes and field effect transistors.     

Intrinsic mobility limit for anisotropic electron transport in Alq3

Muon spin relaxation has been used to probe the charge carrier motion in the molecular conductor Alq3 (tris[8-hydroxy-quinoline] aluminum). At 290 K, the magnetic field dependence of the muon spin relaxation corresponds to that expected for highly anisotropic intermolecular electron hopping. Intermolecular mobility in the fast hopping direction has been found to be 0.23+/-0.03 cm2 V-1 s(-1) in the absence of an electric- field gradient, increasing to 0.32+/-0.06 cm2 V-1 s(-1) in an electric field gradient of 1 MV m(-1). These intrinsic mobility values provide an estimate of the upper limit for mobility achievable in bulk material.     

Electric-field-dependent dispersive transport of carriers in band tails of low-dimensional systems

A theory for the dispersive transport of carriers in the presence of an electric field for the quasi-one-dimensional systems has been developed. It is assumed that localized states are distributed randomly in both space and energy coordinates and that hopping of the carriers occurs in both coordinates. The exponential form of the density of states for band tails is considered. Expressions for the time-dependent demarcation energy and mobility are calculated. The theory predicts that the mobility has three power-law decay branches with exponents nm 1, mm 1 and mm m1. Here nand mare directly proportional to the temperature and electric field respectively. The first power law is associated with the multiple-trapping mechanism and is well known. The other two power laws are new findings of the present theory. The time-of-flight experiments for hydrogenated amorphous SiN x Si/hydrogenated amorphous multilayers can be understood by the present theory on the qualitative basis.     

聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.     

Two-dimensional quantum oscillations of the conductance at LaAlO3/SrTiO3 interfaces

We report on a study of magnetotransport in LaAlO3 /SrTiO3 interfaces characterized by mobilities of the order of several thousands cm2/V s. We observe Shubnikov-de Haas oscillations whose period depends only on the perpendicular component of the magnetic field. This observation directly indicates the formation of a two-dimensional electron gas originating from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass m* ? 1.45 m(e). An electric field applied in the back-gate geometry increases the mobility, the carrier density, and the oscillation frequency.     

掺杂含量对环氧纳米复合电介质陷阱与空间电荷的影响

环氧纳米复合电介质具有抑制空间电荷积聚、高电阻率、高击穿强度等优异性能,对直流电力设备的发展具有重要的作用.但纳米粒子含量对纳米复合电介质陷阱、电导率和空间电荷的影响机理尚不清楚.本文在纳米复合电介质交互区结构模型的基础上提出了计算交互区浅陷阱和深陷阱密度的方法,得到了浅陷阱和深陷阱密度随纳米粒子含量的变化关系.随着纳米粒子含量的增加,浅陷阱密度逐渐增大,深陷阱先增加然后由于交互区重叠的影响而逐渐减少.研究了纳米粒子含量对浅陷阱控制载流子迁移率的影响,发现随着纳米粒子的增多,浅陷阱大幅增多,浅陷阱之间的平均间距迅速减小,导致载流子更容易在浅陷阱间跳跃迁移,浅陷阱控制载流子迁移率增大.建立了纳米复合电介质的电荷输运模型,采用电荷输运模型计算研究了环氧/二氧化钛纳米复合电介质的空间电荷分布、电场分布和电导率特性.发现在纳米粒子添加量较小时,交互区的深陷阱对电导的影响起主导作用;纳米粒子添加量进一步增加,浅陷阱对电导的影响将起到主要作用.     

Effect of magnetic field on Mott's variable-range hopping parameters in multiwall carbon nanotube mat

We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4-150 K and in magnetic fields up to 10 T. It is observed that charge transport in this system is governed by Mott’s variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Mott’s T0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic field-temperature diagram is proposed showing different regions for different kinds of transport mechanism.     

Electronic properties of Semi-Insulating Polycrystalline-Silicon (SIPOS) doped with oxygen atoms

The temperature dependence of the electrical conductivity of SIPOS shows that there are two kinds of conduction mechanisms, conduction in extended states and hopping conduction through localized states dominant above and below room temperature, respectively. The change in the conductance of a SIPOS film due to a transverse electric field can well be understood by the CFO model of amorphous semiconductors and indicates that the Fermi level in the thermal equilibrium is near the midgap, and the results show the density of localized states at the Fermi level to be about 10²⁰ cm^?3 eV^?1.     

Impact ionization of deep acceptor gold in germanium

We observed the dependence of conductivity on an applied high electric field in compensated gold-doped germanium between 77 and 300K. A slight decrease in conductivity, mainly due to the field-dependent carrier mobility, is followed by a relatively large increase in conductivity. The magnitude of this increase depends on the lattice temperature, being larger at low temperatures, and on the compensation ratio. The increase in conductivity is supposed to be due to impact ionization of the frozen-out neutral acceptor gold in the strong electric field. This interpretation is supported by the temperature dependence of the hole concentration in the gold-doped germanium.     

Effect of electric field on diffusion in disordered materials

The effect of electric field on diffusion of charge carriers in disordered materials is studied by Monte Carlo computer simulations and analytical calculations. It is shown how an electric field enhances the diffusion coefficient in the hopping transport mode. The enhancement essentially depends on the temperature and on the energy scale of the disorder potential. It is shown that in one‐dimensional hopping the diffusion coefficient depends linearly on the electric field, while for hopping in three dimensions the dependence is quadratic.     

Dependence of charge injection on temperature, electric field, and energetic disorder in an organic semiconductor

In order to determine energetic disorder's role in facilitating charge injection from gold into a molecularly doped polymer, we have examined the dependence of current on the local electric field, measured using electric force microscopy, at temperatures ranging from 250 to 330 K. From these data we infer, in a single experiment, the temperature dependence of the main factors governing the injection current: the electric-field induced lowering of the image-potential barrier, the interfacial charge density, and the mobility. In this system, the Schottky effect is anomolously large, and the interfacial charge density is larger than expected and strikingly non-Arhennius. Our analysis indicates that these effects are all a consequence of the Gaussian density of states in the organic.     

Structure and electrophysical properties of a lead iron niobate-based amorphous material

An amorphous material based on lead iron niobate (PFN) is studied by X-ray diffraction and dielectric and Mössbauer measurements over a wide temperature range. The atomic structure of amorphous PFN is found to be substantially disordered, which suppresses the transitions into ordered states in the electric and magnetic dipole subsystems that are inherent in crystalline PFN. The dependence of electrical conductivity σ on field frequency ω is shown to correspond to the law σ ～ ω ^s , where parameter s decreases linearly with increasing temperature. This law corresponds to a hopping carrier transfer mechanism that is correlated due to the Coulomb interaction.     

Addendum to “Switching effect and the metal–insulator transition in electric field” by A.L. Pergament et al. [J. Phys. Chem. Solids 71 (2010) 874]

In the paper mentioned above we reported on the switching mechanism in vanadium dioxide which was shown to be based on the electronically-induced Mott insulator-to-metal transition occurring in conditions of the non-equilibrium carrier density excess in the applied electric field, and the proposed model involved the dependence of the carrier density n on electric field (the Poole–Frenkel effect), as well as the dependence of the critical electric field on n. The data on the n(T) dependence were obtained on the assumption of a temperature-independent carrier mobility μ, and the problem of n reduction at lower temperatures was not fully understood. In this Letter we revisit this problem in the light of some recent data on the μ(T) dependence for VO₂. It is shown that the adjusted values of n, taking into account this μ(T), correspond to the Mott critical density within an order of magnitude.     

Kinetics of thermalization of electric charge carriers in organic molecular semiconductors in strong electric fields

The kinetics of photogeneration of electric charge carriers in organic molecular semiconductors in strong electric fields is investigated within a theoretical model that includes the diffusion-drift mechanism of motion of electric charge carriers. In the framework of the proposed model, it is established that the time dependence of the probability of dissociation of a geminal pair exhibits an exponential behavior. The time constant strongly depends on the electric charge carrier hopping distance and the external electric field strength in accordance with the experimental data.     

聚烷基芴电场调制光谱研究

有机发光材料和器件中相关物理机制是一个人们普遍关心的问题, 特别是电场作用下的有机半导体激发态的行为格外引起人们的关注。电场调制发光光谱是研究在电场中元激发的产生和发展的非常有效的方法。文章利用电场调制光致发光(PL)方法研究了聚烷基芴的PL电场猝灭效应,得到聚烷基芴激子离化随电场强度变化的规律和载流子平均跳跃距离。研究还显示PL猝灭因子随激发波长的变化,短波长激发光时猝灭因子大。     

Anisotropic properties and conduction mechanism of TlInSe2 chain semiconductor

TlInSe₂ chain crystals were prepared using the modification of the Bridgman technique. The grown crystals were identified by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). We investigate the anisotropy of transport properties for the first time for TlInSe₂ crystals. Temperature dependence of the dc electrical conductivity, Hall coefficient, Hall mobility, and charge carrier concentration were investigated in the temperature range 184–455 K. The conduction mechanism of TlInSe₂ crystals was studied, and measurements revealed that the dc behavior of the grown crystals can be described by Mott’s variable range hopping (VRH) model in the low temperature range, while it is due to thermoionic emission of charge carriers over the chain boundaries above 369 K. The Mott temperature, the density of states at the Fermi level, and the average hopping distance are estimated in the two crystallographic directions. The temperature dependence of the ac conductivity and the frequency exponent, s, is reasonably well interpreted in terms of the correlated barrier-hopping CBH model.