Predicting morphologies and charge transport in organic photovoltaic polymers
The practice of combining electron rich (donor, D) and electron deficient (acceptor, A) moieties to make low-bandgap co-polymers has led to dramatic efficiency gains in organic photovoltaics (OPV). The flexibility afforded by combining different D and A building blocks to make distinct co-polymers allows for the tuning of the optoelectronic properties of the active layer to improve device performance. While the properties of the active-layer material may be estimated with a variety of electronic structure methods, the interactions between polymer chains, which can significantly impact charge migration, requires simulations in order to estimate morphologies. One avenue for predicting the morphology of candidate materials is via molecular dynamics (MD) simulations using classical force fields. However, accurate force fields for conjugated D-A systems need to be developed. In particular, the torsional potentials in standard force fields have not been parameterized for the sorts of conjugated fused-ring systems typically used in OPV. Incorrect torsional potentials will lead to incorrect amounts of twist along the backbone, and hence incorrect packing between chains. Therefore, torsional potentials for D-A polymers have been explored based on MP2 ab initio calculations. The forms of these potentials as functions of D-A identities and of molecular weight and conjugation length have been investigated and classical force field parameters have been established. We discuss the possibility of creating a transferrable torsional potential for conjugated systems, including what appear to be the limits of such potentials. Atomistic MD simulations of oligomer films for high efficiency D-A copolymers have been performed, allowing for the prediction of packing motifs that have been related to experimental results such as X-ray diffraction, time-resolved microwave conductivity, and device characteristics. Finally, we address implications of these results for extending molecular design of active layer materials beyond isolated-molecule optoelectronic properties to include properties of films.
last modified Nov 02, 2015 08:28 AM