Parameter free calculation of the subgap density of states in poly(3-hexylthiophene)

Abstract

We investigate the influence of intra-chain and inter-chain interactions on the sub-gap density of states in a conjugated polymer using a combination of atomistic molecular dynamics simulation of polymer film structure and tight-binding calculation of electronic energy levels. For disordered assemblies of poly-3-hexylthiophene we find that the tail of the density of hole states is approximately exponential with a characteristic energy of 37 meV, which is similar to experimental values. This tail of states arises mainly from variations in the electronic coupling between neighbouring monomers, and is only slightly influenced by interchain coupling. Thus, knowledge of the disorder in torsion between neighbouring monomers is sufficient to estimate the density of states for the polymer. However, the intrachain torsional disorder is determined largely by the packing of the chains rather than the torsional potential alone. We propose the combination of methods as a tool to design higher mobility conjugated polymers.