Теоретическое описание электронного транспорта в гомо- и молекулярно допированных полимерах
The general aspects of electron transport in disordered organic media (polymers, organic glasses, molecularly doped polymers) are considered. A critical analysis of experimental results and their interpretation in terms of basic theoretical models and experimental methods used in research in this area is presented. Particular attention is given to the comparative analysis of the data obtained by the time-of-flight technique and in studying the radiation conductivity of polymers, in particular, to the possibility of describing the conductivity data in terms of the Gaussian disorder model.
The transient-current curves recorded for molecularly doped polycarbonate are theoretically studied with a version of the time-of-flight technique with a variable thickness of the generation zone. A mixed-carrier-transport model was used, having been proposed in the literature as an alternative to the viewpoint that treats a plateau as an artifact of the optical version of the time-of-flight technique with surface carrier generation. A program for numerical calculation is developed for the model, with the hole transport being dispersive in the generation layer and Gaussian in the rest of the polymer. The model parameter values are selected according to results of independent measurements. It is shown that the mixed-transport model does not explain the experimental observations.
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 mechanism of charge transport in molecularly doped polymers has been the subject of much discussion over the years. In this paper, data obtained from a new experimental variant of the time-of-flight (TOF) technique, called TOF1a, are compared to the predictions of a two-layer multiple trapping model (MTM) with an exponential distribution of traps. In the recently introduced TOF1a experimental variant, the charge generation depth is varied continuously, from surface generation to bulk generation, by varying the energy of the electron-beam excitation source. This produces systematic changes in the shape of the current transient that can be compared to predictions of the two-layer MTM. In the model, one additional assumption is added to the homogeneous MTM, namely: that there exists a surface region, on the order of a micrometer thick, in which the trap distribution is identical to the bulk, but has a higher trap concentration. We find that the characteristic experimental features of an initial spike, a flat plateau, and an anomalously broad tail, as well as the sometimes observed cusp or decreasing current occurring near the transit time, can all be described by such a two-layer model; that is, they can arise as a result of carriers delayed by a trap-rich surface layer. We find that we can semiquantitatively fit current transient data over the whole time range of the experiment, but only by using theoretical parameters that lie in a narrow range, the extent of which we quantify here.
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.