Coarse Grained Model of Conformational Disorder Effects on the Electronic States of Poly(3-hexylthiophene) Chains

Semiconducting polymers are soft materials with many conformational degrees of freedom. Limited understanding of how conformational disorder affects their optical and electronic properties is a key source of difficulties limiting their widespread usage in electronic devices. We develop a coarse-grained approach based on the tight binding approximation to model the electronic degrees of freedom of polythiophene chains, taking into account atomic coordinates changes. Particularly important are dihedral rotations and moiety distortions, which are known to disrupt extended electronic states. The initial stage of our model successfully captures the effects of dihedral rotations on the valence and conduction states of the chains. We extend the model to include the effects of thiophene ring vibrational distortions. We use density functional theory (DFT) to characterize the coupling between the ring normal modes and charge carriers. Our model combined with the variational method is used to predict the formation and the spatial extent of ring-stabilized polarons in poly(3-hexylthiophene). As a way of assessing the relative importance of these polaronic effects to transport, we compute the induced change in carrier effective mass, and compare the polaron size to the carrier localization length resulting from thermal dihedral fluctuations.