Atomistic Simulation of Phase Transitions and Charge Mobility for the Organic Semiconductor Ph-BTBT-C10

We present a computational study of the molecular organization and charge mobility of Ph-BTBT-C10, a high performance organic semiconductor of considerable current interest. We have observed for the first time by atomistic molecular dynamics the formation of an ordered smectic phase on cooling down from the isotropic melt and upon heating the crystal, for this system, in good agreement with experiment. Although we could observe only a smectic A and not a smectic E phase, the temperature variation of the hole mobility estimated from hopping model calculations reproduces the main features of experiments. The crystal phase is characterized by high mobility bilayers defined by the aromatic π-conjugated cores, but it is effectively insulating in the orthogonal direction. The smectic phase is characterized by more disordered monolayers, which have a good in-plane mobility and a lower, but still appreciable, degree of charge transport across the layers. This feature may be advantageous for applications requiring materials with evenly balanced, three-dimensional conduction paths.