Dilution and cluster contributions to hopping transport in a bias field

Sin, J. M.; Soos, Z. G.
June 2002
Journal of Chemical Physics;6/1/2002, Vol. 116 Issue 21, p9475
Academic Journal
Hole transport in molecularly doped polymers (MDPs) is modeled as random walks on fixed donors (Ds) embedded in a polymer matrix. Dilution p < 1 corresponds to placing individual Ds, dimers D[sub 2], or tetramers D[sub 4] randomly on a fraction p of sites in a face-centered-cubic lattice. Monte Carlo simulations of the drift velocity υ[sub D](E) in a bias field E have maxima in dilute (p = 8%) systems of D[sub 2] or D[sub 4] that are related to the formation and polarization of clusters of nearest-neighbor donors. Marcus or small-polaron hopping rates with fixed parameters account for the concentration, field, and temperature dependencies of the mobility, µ(E, T) = υ[sub D](E, T)/E, of D = TTA (tritolylamine) in polystyrene and of related systems with D[sub 2] or D[sub 4] in PS. The compensation temperature is lower for D[sub 2] than for D at p = 20%, consistent with stronger positional disorder for dimers. The anomalous broadening of photocurrents in D[sub 4] with increasing E is due to cluster polarization. The parameter σ = 700 K for energetic disorder is used throughout and is ∼25% smaller than in the Gaussian disorder model. Spatially correlated energies yield the characteristic field dependence of µ(E, T). Although not quantitative, the comprehensive treatment of dilution in TTA:PS and related MDPs clearly supports Marcus hopping rates and stronger geometrical than energetic disorder.


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