  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.
