Predicting structure and charge transport in semiconducting polymers

Organic semiconducting polymers for photovoltaics are improving in their performance, but a nanoscale picture of these materials is lacking. What does the donor-acceptor interface look like? What do free charges look like? And how do charges move, along and between chains? These are theoretically challenging questions, for which we need both simulations and quantum calculations. But simulations are hard, because these molecules are bulky and slow to move. DFT calculations are also hard, in part because the system is disordered.

To explore the interface, we need atomistic simulations, fast enough to equilibrate mixtures over many microseconds. We achieve this by taking advantage of the stiffness of rings to greatly reduce degrees of freedom, while retaining atomistic structure. Equilibrated donor-acceptor amorphous interfaces are a few nanometers wide, consistent with simulation results for chi using our recently developed pulling technique. Acceptors are somewhat soluble in the donor phase, which may trap excitons and reduce performance.

To describe charges and how they move, we use tight-binding models parameterized by comparison to DFT calculations on monomers and dimers. Free charges are present as polarons, stabilized by electronic polarization of the surrounding material. Polarons move along chains in an energy landscape determined by dihedral disorder; hops between chains are described by Marcus theory. We assemble these ingredients without adjustable parameters to predict polaron mobility in amorphous P3HT, and find promising agreement with experiment.