A long standing issue for transport in molecularly doped polymers is the compatibility of the small polaron hopping rate. It has been recently shown that the Poole-Frenkle field dependence of the mobility of injected charges in molecularly doped polymers arises as a natural consequence of the charge-dipole interaction. In this paper we address the extent to which the Poole-Frenkle behavior is compatible with the dependent upon the size of the polaron binding energy.
We present results of computer simulations of the charge carrier mobility dependence on the electric field and trap concentration in disordered polar organic materials. Recently an unusual dependence of the charge carrier drift mobility in molecularly doped polymers on the concentration of traps has been reported. This dependence differs from the expected inverse proportionality that should be valid for trap-controlled transport. Using our results we argue that this dependence is caused by the existence of different regimes of charge carrier transport for layers with different trap concentrations, i.e. dispersive transport for small trap concentrations and nondispersive transport for high trap concentrations. Our results also show that the mobility, estimated from the time of intersection of the asymptotes to plateaus and trailing edge of photocurrent transient, is very sensitive to variation of transient shape and, in some case, effectively masquerades real concentration and field dependence of the true mobility. Change of the shape with the change of electric field in trap-containing matrices distorts significantly the field dependence of the mobility, calculated by the intersection procedure, and even transforms the usual Poole-Frenkel dependence to the linear one.
KEYWORDS: Molecules, 3D modeling, Organic materials, Polymers, Correlation function, Computer simulations, Information operations, Glasses, Monte Carlo methods, Matrices
The reason for the Poole-Frenkel (PF) mobility field dependence in dependence in disordered organic materials is believed to be a long range spatial correlation in the distribution of enough levels of transport sites. This correlation is produced by molecules with significant permanent dipole moments. However, experimental data offer a strong evidence that in transport layers containing no molecules with high dipole moment essentially the same PF dependence of transport parameters on the mean intersite separation. By means of computer simulation and analytic calculation we consider the influence of deep traps on the charge carrier mobility in organic materials.
Aspects of dynamic disorder in charge transport in polymers are investigated. Basic issues such as polaron formation, the validity of semiclassical arguments and the memory function method of mobility calculation are studied.
The interpretation of scanning tunneling microscopy (STM) images of adsorbed molecules requires a redefinition of the current, that goes beyond the limitations imposed by the Transfer Hamiltonian formalism. Here we discuss two expressions for the tunneling current we derived, both suitable for situations in which an adsorbate is placed between tip and substrate. The former is still based on a perturbative approach, but emphasizes the contributions to the STM current due to the adsorbate. The latter is an exact expression obtained by treating the STM current like an electron transport process. This allows the inclusion of both the quantum aspects of the transport and thermal effects, and it can be used in widely different situations.
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