Dipolar disorder formalism revisited
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.
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]
The effect of preliminary electron beam irradiation on hole transport in a molecularly doped polymer was studied with the use of the time of flight technique in the radiation_induced mode. Specimens that exhibit a plateau on their time of flight curves were selected for the study, since they suggest the occurrence of quasi equilibrium transport in the system according to the conventional point of view. In the extremely small signal mode, current transients in the case of bulk irradiation have a form corresponding to dispersive, rather than Gaussian, transport, although hole movement is observed in the presence of charged sites (trapped electrons). On passing to the moderately large signal mode (preirradiation to a dose of up to 5 Gy), the current transients undergo noticeable changes, which might be mistakenly interpreted as evidence for the influence of charged sites on hole transport in accordance with the predictions of the dipolar glass theory. In actuality, these changes are due to the effect of a space charge field and the hole mobility remains almost unchanged in this case. The appearance of the plateau on the current transients is an artifact of the procedure, and the hole transport is dispersive.
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.
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.