Simulating charge transport in tris(8-hydroxyquinoline) aluminium (Alq ₃)

We present a model of charge transport in organic solids which explicitly considers the packing and electronic structure of individual molecules. We simulate the time-of-flight mobility measurement in crystalline and disordered films of tris(8-hydroxyquinoline) aluminium (Alq₃). The morphology of disordered Alq₃ is modelled on a molecular scale, and density functional theory is used to determine the electronic couplings between molecules. Without any fitting parameters we predict electron mobilities in the crystalline and disordered phases of ∼1 and ∼10^-4 cm ² V^-1 s^-1, respectively. In good agreement with experiment we find that electron mobilities are two orders of magnitude greater than those of holes. We explain this difference in terms of the spatial extent of the frontier orbitals. Our results suggest that charge transport in disordered Alq₃ is dominated by a few highly conducting pathways.