Nanoscale Orientation Effects on Carrier Transport in a Low-Band-Gap Polymer

Abstract

We show that due to a substrate-induced orientation of the local morphology of thin supported conjugated polymer films of poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexan-1-one)thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-C), the out-of-plane hole mobility, μ, increased with increasing film thickness h, becoming independent of h for h > 110 nm. The temperature and electric field dependencies of μ are in agreement with predictions of the well-known Gaussian Disorder Model (GDM), developed to describe charge transport in materials possessing positional and energetic disorder. Room temperature studies reveal a negative dependence of μ on the electric field E, with a strength quantified by the parameter β. The largest magnitude of β was measured in the thinnest films, indicative of the strongest dependence of μ on the electric field E; β decreased monotonically with increasing film thickness. The thickness dependencies of μ and β manifest an increase of the average anisotropy of the films with decreasing film thickness, corroborated by UV–vis spectroscopy and spectroscopic ellipsometry measurements.