Abstract—General questions about hole transport and bimolecular recombination of charge carriers in molecularly doped polycarbonate with a low dopant concentration (10 wt %) are considered. The experiment is performed via a radiation-induced time-of-flight technique with bulk generation of charge carriers. Transient-current curves are calculated numerically via a multiple-trapping model. There is good agreement between the calculated and experimental results on the transient-current waveform. Nonequilibrium hole transport is observed in the studied molecularly doped polymer, and the bimolecular recombination is close to the Langevin recombination as described by the multiple-trapping model.
With the use of n -triacontane models as examples, abnormal characteristics of diffusion that manifest themselves during the application of the Einstein–Smoluchowski relationship and the asymptotic behavior of velocity autocorrelation function of the molecule-mass centers that is used to calculate the diffusion coefficient via the Green–Kubo formula are investigated. On the basis of the data of complementary approaches, the microscopic mechanisms of diffusion in higher alkanes are outlined. The applicability of the Stokes–Einstein relationship for the viscosity coefficient is demonstrated.
Abstract—Transient current curves in a polar molecularly doped polymer for subsurface and bulk generations of charge carriers are compared. No transformation of the horizontal plateau on the former to the curve predicted by the diffusion and drift theory was found, contrary to expectations for quasi-equilibrium transport conditions. This behavior indicates a nonequilibrium character of the transport of charge carriers. The plateau appearance is explained in our case by the depleted subsurface layer effect, as we already pointed out earlier.
Carried out direct experimental comparison of the field dependence of the drift and the effective mobility of holes in the polar molecularly doped polymer, the polycarbonate containing 30 wt. % diphenylhydrazone -diethylaminobenzaldehyde) based on the method of time-of-flight in volumetric irradiation samples of polymer with a pulse of fast electrons. Numerical simulation of time-of-flight experiment was performed using the multiple trapping model with the Gaussian trap distribution in energy. The model parameters are determined from the independent measurements. It is shown that due to the nonequilibrium transport of holes in terms of time-of-flight experiment the true constant of the Poole–Frenkel (0.27 (μ/IV)^1/2) is only part of the experimentally determined value (0.39 (μ/IV)^1/2)
Abstract—The multipletrapping model with a Gaussian trap distribution was used for the numerical analysis of the effects of nonequilibrium transport on the temperature dependence and field dependence of the mobility of carriers in a typical molecularly doped polymer. It was shown that, with allowance for the prescribed Poole–Frenkel Frenkel type field dependence of mobility under the conditions of nonequilibrium transport, the observed decrease in the effective energy of disorder may be explained as a compensation effect with no change in the initial energy distribution of hopping centers.
Abstract—The charge-carrier transport model based on the multiple-trapping quasi-band theory with the Gaussian or exponential energy distributions of traps for the two-layer structure of the molecularly doped polymer sample is used to explain the experimentally observed constancy of the flat plateau on time-of-flight curves in a wide electric-field range. The constant shape of the time-of-flight curve under the condition of nonequilibrium transport is observed for the exponential, rather than Gaussian, energy trap distribution. This observation may be used to distinguish between these trap distributions. Finding the true Poole–Frenkel constant requires that the nonequilibrium transport in molecularly doped polymers be taken into account during treatment of the data on the field dependence of mobility.
Abstract—The general problems of the bimolecular recombination of charge carriers in molecularly doped polycarbonate are discussed. Experimental studies are performed via the transient radiationinduced conductivity method. Transientcurrent curves are numerically calculated via the multiple trapping model. The calculated and experimental curves are in good agreement. It is shown that the studied molecularly doped polymer undergoes bimolecular recombination via the Langevin mechanism.
Abstract—General problems of bimolecular recombination of charge carriers in both pure and molecularly doped branched polyphenylenevinylenes are addressed. Experiments are performed via the nonstationary radiationinduced electricalconductivity method. Transientcurrent curves are numerically calculated in terms of the multipletrapping model. Good agreement between the calculated and experimental curves is attained. In the investigated polymers, the Langevin mechanism of bimolecular recombination is active.
Abstract — The effect of charged centers on charge-carrier mobility in polycarbonate, a polar molecularly doped polymer, is studied. The nature of this effect is revealed, and a simplified physicomathematical model is proposed to describe it. The performed numerical calculations are qualitatively consistent with experimental results. Preliminary studies are conducted to elucidate the nature of the defective surface layer in molecularly doped polymer samples
With the use of n-triacontane models as examples, abnormal characteristics of diffusion that manifest themselves during the application of the Einstein–Smoluchowski relationship and the asymptotic behavior of velocity autocorrelation function of the molecule-mass centers that is used to calculate the diffusion coefficient via the Green–Kubo formula are investigated. On the basis of the data of complementary approaches, the microscopic mechanisms of diffusion in higher alkanes are outlined. The applicability of the Stokes–Einstein relationship for the viscosity coefficient is demonstrated.
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.
Abstract - A program for numerical computation of the curves of the transient current in samples of molecularly doped polymers has been developed with allowance for the presence of a defective surface layer in them. Time-of-flight curves are calculated with the multiple-trapping model with a Gaussian energy distribution of traps. The model parameters are determined from the results of independent measurements. Numerical computations are in qualitative agreement with experimental data for a typical molecularly doped polymer. The features of the formation of a flat plateau in samples of different thickness are discussed in terms of the proposed model.
The shape of time-of-flight curves in the mode of subsurface irradiation of samples with lowenergy electrons is analyzed for free_standing films of a typical molecularly doped polymer of different thicknesses (11–45 μm). Special attention is paid to comparison of curves registered for both sides of the samples. The data confirm the hypothesis that the defective layer is formed owing to sublimation of dopant molecules during sample preparation and qualitatively agree with predictions of the two_layer multiple_trapping model.29.19.23 Теория электрических свойств твердых тел 29.19.25 Взаимодействие проникающего излучения с твердыми телами
Abstract — A multiple-trapping model has been used to numerically analyze the effects of the nonequilibrium transport of holes on the field dependence of drift mobility as obtained in a time-of-flight experiment. The intrinsic field dependence has been assigned to the frequency factor in the form of the Poole–Frenkel law. Strongly dispersive transport, as well as a nonequilibrium-charge-transfer regime given by the Gaussian disorder model, has been considered. It has been established that the currently accepted approach to the analysis of the field dependence of mobility needs to be reexamined.