Model of diffusion-drift charge carrier transport in layers with a fractal structure

A new theoretical (semiempirical) model of diffusion-drift charge carrier transport in layers with a fractal structure based on a partial differential equation with a fractional time derivative has been proposed. It has been shown by numerical calculations that a decrease in the order of the fractional derivative in the presence of the external electric field leads to broadening and asymmetry of the spatial distributions of charge carriers, which physically corresponds to intensification of scattering processes.     

Quadrupolar glass as a model for charge carrier transport in nonpolar organic materials

Monte Carlo simulation of the charge carrier transport in disordered nonpolar organic materials has been carried out. As a suitable model we considered the model of quadrupolar glass. A general formula for the temperature and field dependence of the mobility was suggested. A comparison with experimental data has been carried out.     

Charge carrier transport in conducting polymers on the metal side of the metal-insulator transition: A review

The advances achieved in the field of the synthesis of conducting polymer films and the results of experimental investigations of the mechanism of charge carrier transport in heavily doped conducting polymer films on the metal side of the metal-insulator transition have been considered. The samples studied belong to the last generation of conducting polymers with a low degree of structural disorder, which makes it possible to dope samples to a highly conducting state characterized by an increased stability. Apart from the study of the morphology, structure, and doping conditions, the mechanism of low-temperature transport in these polymers has been investigated in detail. The results of investigations of the transport at low temperatures (T < 1 K) have been described, and a phenomenological model of charge carrier transfer in these systems has been proposed. The polymers under investigation have been widely used as injecting and accumulating layers in light-emitting organic diodes and solar cells.     

Effects of contact resistance on the evaluation of charge carrier mobilities and transport parameters in amorphous zinc tin oxide thin-film transistors

Accurate determination of the charge transport characteristics of amorphous metal-oxide transistors requires the mitigation of the effects of contact resistance. The use of additional electrodes as voltage probes can overcome contact resistance-related limitations and yields accurate charge carrier mobility values, trap depths and temperature and carrier density dependencies of mobility as well as trap depths. We show that large differences in measured charge carrier mobility values are obtained when such contact resistances are not factored out. Upon exclusion of the contact resistance, the true temperature dependence of charge carrier mobility appears in the form of two clearly distinct mobility regimes. Analyzing these revealed mobility regions leads to a more accurate determination of the underlying transport physics, which shows that contact resistance-related artefacts yield incorrect trends of trap depth with gate voltage, potentially leading to a misconstruction of the charge transport picture. Furthermore, a comparison of low- and high-mobility samples indicates that the observed effects are more general.     

Investigation of MWS Relaxation in Nylon 1010 using Dielectric Relaxation Spectroscopy

Maxwell–Wagner–Sillars (MWS) relaxation in nylon 1010, arising from charge carriers accumulated at the interphase between amorphous and crystalline regions, has been investigated by means of dielectric relaxation spectra. In the frequency spectra of nylon 1010, dielectric permittivity showed high values at low frequencies originating from charge carrier movement. For the MWS relaxation, the dielectric strength was independent of temperature. The results revealed that there is a transition temperature, located between 110 and 120°C, resulting in the separation of two different charge carrier movement mechanisms. Below and above this transition temperature, the temperature dependence of the MWS relaxation time follows the Vogel–Tammann–Fulcher type, showing that the charge carrier transport is governed by the motion of the polymer chains. The change of charge carrier movement mechanisms is due to the onset of polymer chain motion in the interphase.     

Voltage-induced metal-insulator transition in polythiophene field-effect transistors

We have extensively studied the carrier transport in regio-regular polythiophene field-effect transistors (FETs) from room temperature to 4.2 K. At low temperatures, Zabrodskii plots (dlnsigma/dlnT) demonstrate that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition at a carrier density of 5x10(12) cm-2. The carrier transport in the insulating regime is well described by phonon assisted hopping in a disordered Fermi glass with Coulomb interaction between the hopping charge carrier and the opposite charge left behind, as described by Efros and Shklovskii.     

Study of the charge carrier transport through a metal-poly(arylenephthalide)-class polymer interface

The influence of the chemical structure of the poly(arylenephthalide)-class polymers on the charge carrier transport through a metal/polymer interface has been discussed. On the basis of the results obtained by physical and optical methods and quantum chemical modeling, the conclusion is made that a change in the skeleton part of the polymer molecule leads to a substantial change in the value of the potential barrier.     

Bimolecular recombination coefficient as a sensitive testing parameter for low-mobility solar-cell materials

Bimolecular charge carrier recombination has been clarified in bulk-heterojunction solar cells based on a blend of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene using the time-of-flight method. We show how bimolecular recombination influences the charge carrier transport, how it limits the efficiency of low-mobility solar cells, and how to estimate the bimolecular recombination coefficient. We found that bimolecular recombination in these efficient photovoltaic materials is orders of magnitude slower as compared to Langevin recombination expected for low-mobility materials. This effect is inherent to the nanomorphology of the bicontinuous interpenetrating network creating separate pathways for electrons and holes, and paves the way for the fabrication of bulk-heterojunction solar cells where bimolecular recombination is not the limiting factor.     

Photoinduced transient stark spectroscopy in organic semiconductors: a method for charge mobility determination in the picosecond regime

Subpicosecond photoinduced Stark spectroscopy experiments are carried out for measuring charge carrier mobility in organic semiconductors. The technique is demonstrated in state-of-art devices based on methanofullerene. The transient mobility of photogenerated charge carriers is measured in the picosecond time domain. Electric field dependent mobility is observed from the earliest time scales. In addition, two distinct transport regimes are revealed: a short-lived state, approximately 10 ps, of high mobility and a transient towards the trap limited transport, associated with the mesoscopic structure of the medium.     

Polaron effect dependence of thermopower in organic semiconductors

A unified physical model for thermopower was presented in organic semiconductors, based on the Marcus theory and variable-range hopping theory. According to the proposed model, the characteristic of charge carrier thermoelectric transport in organic semiconductors has been investigated. In particular, polaron effects, energetic disorder, and carrier density dependence of the thermopower have been discussed in detailed. The calculation also shows a good agreement with the experimental data in organic semiconductors.     

Ultrapure,high mobility organic photoconductors

This contribution demonstrates that high charge carrier mobility (<400 cm²/Vs) is an inherent property ofultrapure organic molecular crystals at low temperatures. Small concentrations of traps, however, can completely obscure these microscopic transport properties on macroscopic scales. We describe extensive purification procedures with naphthalene and perylene, which led to the observation of high mobilities. At the same time we demonstrate that charge carrier transport measurements are a sensitive tool for the analytical characterization of high purity organic molecular crystals.This work has been presented in part at the VIIth Intern. Conf. on Crystal Growth, Stuttgart (1983)     

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.     

Charge carrier energy disorder in polar amorphous organic thin films

We report Monte Carlo calculations of the charge carrier energy level disorder in polar amorphous organic solids arising from electrostatic interactions between polarizable molecular charge distributions. In contrast with previous reports in which the discrete molecular polarizibilities are modeled as a dielectric continuum having dielectric constant epsilon, our calculations take account of the molecular-scale polarization inhomogeneity. We find that the previously reported 1/epsilon scaling rule underestimates the charge carrier energy level disorder by more than a factor of 2 for typical values of epsilon. Such an underestimation strongly impacts the analysis of charge carrier transport in polar amorphous organic thin films, due to the strong dependence of the charge carrier mobility on the energy disorder.     

Charge-carrier and polaron hopping mobility in disordered organic solids: Carrier-concentration and electric-field effects

The effective-medium approximation (EMA) analytical theory is advanced further to describe charge transport at arbitrary charge-carrier concentration in a disordered organic material with superimposed polaron effects. A key point of this model compared to the previous treatment [Phys. Rev. B 76 (2007) 045210] is that it is formulated for arbitrary electric fields and is able to describe consistently both the carrier-concentration and field dependences of charge mobility. The mobilities of both bare charge carriers and polarons were calculated using the Miller–Abrahams and polaron jump rate models, respectively. An excellent quantitative agreement was obtained between the theoretical calculations and the recent numerical simulations of the field- and carrier-density dependences of the mobility for bare charge carriers using the same parameters. The polaronic carrier density effect was also calculated using the complete Marcus jump rate equation and straightforward EMA configurational averaging, and the results compared to that obtained with the use of the symmetrical jump rate model and the effective transport energy concept. This study confirms that a strong dependence of carrier mobility upon increasing carrier density and electric field, which has conventionally been observed in experiment for numerous organic semiconducting materials, is incompatible with the notion of large polaron binding energy in these materials, implying that the energetic disorder plays a dominant role.     

Influence of light on the bulk charge in a dielectric under exposure to electrons

Singularities of the charge transfer during exposure of a dielectric with an open surface to an electron beam in the presence of light are investigated. Within the framework of a quasi-free carrier models a general expression is obtained that describes the transition potential of the open surface under a jumplike change in exposure conditions. Experiments are conducted displaying the applicability of the quasi-free carrier model to charge transport under conditions of multiple trapping and preferred photoliberation of carriers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 62–67, November, 1989.     

DC column plasma kinetics in a longitudinal magnetic field

The cylindrical column plasma of a neon dc glow discharge under the influence of a weak longitudinal magnetic field is studied. An extended, fully self-consistent model of the column plasma has been used to determine the kinetic quantities of electrons, ions and excited atoms, the radial space charge field, and the axial electric field for given discharge conditions. The model includes a nonlocal kinetic treatment of the electrons by solving their spatially inhomogeneous kinetic equation, taking into account the radial space charge field and the axial magnetic field. The treatment is based on the two-term expansion of the velocity distribution and comprises the determination of its isotropic and anisotropic components in the axial, radial, and azimuthal direction. A transition from a distinctly nonlocal kinetic behavior of the electrons in the magnetic-field-free case to an almost local kinetic behavior has been found by increasing the magnetic field. The establishment of the electron cyclotron motion around the column axis increasingly restricts the radial electron energy transport and reduces the radial ambipolar current. The complex interaction of these transport phenomena with the alterations in the charge carrier production leads finally to a specific variation of the electric field components. The axial field increases by applying weak magnetic fields, however, decreases with increasingly higher magnetic fields. At higher magnetic fields, the radial space-charge field is considerably reduced     

Universal crossover from band to hopping conduction in molecular organic semiconductors

The charge transport in a variety of herringbone-stacked organic molecular semiconductors is investigated in the temperature range from 10 to 550 K. A crossover from coherent bandlike charge transport with mobilities up to several thousand cm (2)/V s at low temperature to an incoherent hopping motion at high temperatures is observed. This is attributed to the localization of the charge carrier due to increased electron-phonon interaction and, finally, the formation of a lattice polaron.     

考虑空间电荷层效应的氧离子导体电解质内载流子传输特性

晶界或异质界面诱发的空间电荷层(space charge layer,SCL)效应,被认为是氧离子导体电解质内界面附近区域载流子传输特性显著区别于体相区域的关键原因之一.现有研究多采用Poisson-Boltzmann(PB)方程预测SCL效应的影响规律,但其基于载流子电化学平衡假设,无法用于载流子存在宏观运动的工况,极大限制了相关传输机理研究.本文耦合Poisson方程和载流子质量守恒方程,建立了适用于载流子具有宏观运动时氧离子导体内载流子传输过程的模型,推导了控制SCL效应的关键无量纲参数.聚焦固体氧化物燃料电池中常用的AO2-M2O3氧离子导体电解质,对比研究了传统PB方程和本文建立的Poisson-载流子质量守恒耦合方程的预测结果可靠性.进一步采用耦合模型深入分析了考虑SCL效应时氧离子导体内部氧空位传输机理,发现导体界面电流密度增大导致SCL电阻先减小后增大.增大无量纲Debye长度(表征空间电荷层厚度与导体厚度的比值)可显著增大SCL电阻.当驱动氧空位移动的过电势与热势数量级相当时,增大无量纲电势(表征过电势与热势的比值)导致SCL电阻增大;当过电势远小于热势时,改变无量纲电势对氧空位传输过程几乎无影响.本文研究结论可为通过合理设计晶界或异质界面以改善氧离子导体内载流子传输能力及最终提高相关电化学器件性能提供理论依据.     

有机半导体异质界面电荷传输解析模型研究

有机半导体多层薄膜器件的性质很大程度上由有机-有机界面的传输性质所决定，但是现有的关于有机-有机界面的分析模型很难适用于实际器件的模拟.以Miller-Abrahams跳跃传导理论为基础，充分考虑有机-有机界面和金属-有机界面性质的不同，建立了一个新的描述有机-有机异质界面电荷传输的解析模型.结果表明有机异质界面的载流子传输不仅取决于界面的肖特基势垒，而且还取决于界面附近两边的电场强度和载流子浓度.此模型可用于有机半导体多层薄膜器件的电流密度、电场分布和载流子浓度分布的自洽计算. 关键词： 有机半导体 界面 载流子传输