Abstract
Abstract Understanding the charge transport in molecular semiconductor mixtures remains challenging, largely due to the lack of a universal dependence of carrier mobility upon doping concentration. Here we demonstrate that it is feasible to use the percolation theory to explain the change of charge mobility in a model system of 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) and tris-(8-hydroxyquinoline) aluminum (Alq3) with various doping concentrations. As the fraction of CBP within the mixtures increases, the charge mobility firstly shows a reduction at low CBP fraction due to the scattering effect, and then increases well following a percolation model. Electron microscopy and atomic force microscopy analysis suggest that CBP and Alq3 are homogeneously mixed in their co-evaporated amorphous films, which meets the precondition for using percolation theory. We describe the possible microcosmic percolating mechanism with a model combining bond percolation with charge transfer integral calculation. Based on this model, the percolation threshold in molecular semiconductor mixtures can be predicated. For the hole and electron transport in our system, the predicated percolation thresholds are very close to the experimental values.
Published Version
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