Charge carrier transport in molecularly doped polycarbonate as a test case for the dipolar glass model
We present the results of Monte Carlo simulations of the charge carrier transport in a disordered molecular system containing spatial and energetic disorders using the dipolar glass model. Model parameters of the material were chosen to fit a typical polar organic photoconductor polycarbonate doped with 30% of aromatic hydrazone, whose transport properties are well documented in literature. Simulated carrier mobility demonstrates a usual Poole-Frenkel field dependence and its slope is very close to the experimental value without using any adjustable parameter. At room temperature transients are universal with respect to the electric field and transport layer hickness. At the same time, carrier mobility does not depend on the layer thickness and transients develop a well-defined plateau where the current does not depend on time, thus demonstrating a non-dispersive transport regime. Tails of the transients decay as power law with the exponent close to −2. This particular feature indicates that transients are close to the boundary between dispersive and non-dispersive transport regimes. Shapes of the simulated transients are in very good agreement with the experimental ones. In summary, we provide a first verification of a self-consistency of the dipolar glass transport model, where major transport parameters, extracted from the experimental transport data, are then used in the transport simulation, and the resulting mobility field dependence and transients are in very good agreement with the initial experimental data. © 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794791]
Abstract—A theoretical analysis of the nonlinear field dependence of the pulsed radiationinduced conductiv ity of polystyrene in strong electric fields is performed. An explanation of the previously uncomprehended phe nomenon of the sharp increase in the radiationinduced electrical conductivity in the presence of an electric field with a strength of above 5 × 107 V/m is proposed. This behavior cannot be explained within the framework of Onsager’s theory of the fieldassisted thermal production of free charges from generated geminate pairs.
Our approach to the problem of charge carrier transport in homogeneous polymers assumes carrier hopping on a densely packed manifold of transport (hopping) centers retaining the short-range order of the respective single crystal. The central idea, which distinguishes it from the Bassler’s disorder theory, is that the energy scatter concerns only a small fraction of hopping centers, which begin to act as traps. The majority of isoenergetic centers build up a transfer band with microscopic mobility µ0 equal to that in the respective single crystal. The origin of the energy scatter (and traps themselves) is ascribed to the elementary voids of the fluctuation free volume forming an association with the normal hopping centers.
The dipolar disorder formalism (DDF) of Borsenberger and Bдssler has been further developed based on a unified approach treating the van der Waals and the dipolar disorder energies as being roportional to mean intersite distance in a certain power. Tested against real molecularly doped polymers with the concentration of the dopant changing in a wide range, this approach gives values of the exponent lying in the interval from _1.5 to _2.5. The total disorder is represented by an algebraic combination of four material parameters relating to the dopant and the polymer matrix weighted by their relative weight concentrations. What is important, we seem to get able to explain the near constancy of the total disorder when the concentration of the polar dopant changes. Until recently, this unusual behavior of the total disorder defied any reasonable explanation.
The time-of-flight technique is used to measure hole mobility in molecularly doped polycarbonate and polystyrene that contain both polar and weakly polar additives. The two versions of the technique with the bulk and surface generation of charge carriers under small-signal conditions are employed. Numerical calculations show that the time dependence of the transient-current curves obtained with the first version of the technique is in agreement with the theory of multiple trapping for an exponential energy distribution of traps. In the case of time-of-flight curves with surface generation, the run of the post-transit branch is likewise consistent with the theory, whereas this consistency is often violated for the pretransit branch of the curves. This result is due to the effect of the defective surface layer of a polymer, which is not taken into account in numerical calculations. The results show that the hole transport in the studied molecularly doped polymers is dispersive. An increase in the polarity of the polymer matrix and the dopant drastically decreases the hole mobility and, at the same time, increases its field and temperature dependence.
The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.
Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.
This volume presents new results in the study and optimization of information transmission models in telecommunication networks using different approaches, mainly based on theiries of queueing systems and queueing networks .
The paper provides a number of proposed draft operational guidelines for technology measurement and includes a number of tentative technology definitions to be used for statistical purposes, principles for identification and classification of potentially growing technology areas, suggestions on the survey strategies and indicators. These are the key components of an internationally harmonized framework for collecting and interpreting technology data that would need to be further developed through a broader consultation process. A summary of definitions of technology already available in OECD manuals and the stocktaking results are provided in the Annex section.