Monte Carlo simulation of charge transport is employed to test the suitability of extensively used empirical equations, proposed by Gaussian disorder model (GDM)/ correlated disorder model (CDM), for extracting disorder parameters from the field and temperature dependence of mobility in inhomogeneous organic thin films. Numerous observations suggests that the effective energetic disorder seen by the carrier in an inhomogeneous system decreases with increase in concentration of low disordered regions. However, the conventional analysis of field and temperature dependence of mobility obtained for such system, even with high concentration of low disordered regions, using the empirical equation provides the value of energetic disorder that is close to the value for regions of high disorder in the system rather than the effective energetic disorder. The study also shows that the inaccurate value of energetic disorder extracted using empirical equation in turn affect the extraction of positional disorder parameter. Possible reason for inaccurate estimation of disorder parameters is discussed. We conclude that a modified approach is indispensible to obtain effective disorder parameters for an inhomogeneous system.

The influence of ordered regions (micro crystallites and aggregates) in the other wise disordered polymer host matrix on field and temperature dependence of mobility (μ) has been simulated.Increase in concentration of ordered regions leads to increase in magnitude of mobility and in high field regime the saturation of the mobility occurs at lower electric field strength. The influence of different mean and standard deviation of Gaussian density of states (DOS) of ordered regions on the field dependence of mobility was studied and found to be significant only at higher concentrations. Weak influence of these parameters at low concentrations are attributed to the strong interface effects due to the difference in the standard deviation of DOS of two regions (host and ordered region) and shallow trapping effect by ordered regions. For all the parameters of ordered regions under investigation the temperature dependence of mobility (logμ) and the slope of logμ Vs E 1/2 plot show 1/T 2 dependence.

Monte Carlo simulation was carried out to understand the influence of morphological inhomogeneity on carrier diffusion in organic thin films. The morphological inhomogeneity was considered in the simulation by incorporating the regions of low energetic disorder in a host lattice of high energetic disorder which decreases the overall energetic disorder of the system.For the homogeneous films, the carrier diffusion was found to decrease upon decreasing the energetic disorder. In contrast to this, in the case of inhomogeneous films the carrier diffusion enhanced upon decreasing the overall energetic disorder, up to an optimum value and beyond which the carrier diffusion decreased. Through our simulation, we observed that the behavior of carrier diffusion in the inhomogeneous case is due to the morphology dependent carrier spreading, which acts in addition to the thermal and non-thermal field assisted diffusion mechanisms. This morphological dependence of carrier spreading arises due to the generation of packets of carriers with different jump rates, which is after effect of slow relaxation of the carriers generated in the less disordered regions of inhomogeneous system. Our simulation of morphology dependent carrier spreading and its influence on the basic diffusion process provide deeper insight into the charge transport mechanisms in organic thin films.

A total negative field dependence of hole mobility down to low temperature was observed in N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine (TPD) doped in Polystyrene. The observed field dependence of mobility is explained on the basis of low values of energetic and positional disorder present in the sample. The low value of disorder is attributed to different morphology of the sample due to aggregation/crystallization of TPD. Monte Carlo simulations were also performed to understand the influence of aggregates on charge transport in disordered medium with correlated site energies. The simulation supports our experimental observations and justification on the basis of low values of disorder parameters.

No abstract