Crystalline and Amorphous Interface Simulations of Donor-Acceptor Blends.

Abstract

In an organic solar cell, exciton dissociation and charge transport to generate current depend on interface and bulk morphology, respectively, and their rates dictate device performance. Blend miscibility and processing determine the final morphology. We investigate the blend miscibility of P3HT and O-IDTBR, employing our recently developed "push-pull" computational technique, and explore its effect on nanoscale morphology. The resulting Flory-Huggins χ parameter is primarily enthalpic, and the blend exhibits UCST behavior. To directly simulate an equilibrated morphology in such a blend requires large (30 nm) systems and long (10 μs) simulations, which we attain with our virtual site coarse graining technique. In the resulting amorphous phase-separated state, O-IDTBR swells P3HT by about 15%, but does not percolate within the P3HT-rich phase. The interfacial profile depends on crystallinity: the amorphous interface is several nanometers wide, whereas an acceptor crystal induces order in the adjacent donor polymer, forming a sharp interface quite similar to a crystal-crystal interface. The wide amorphous interface promotes more donor-acceptor contacts with a wide range of relative orientations; however, the acceptors do not form a percolating network, which will likely lead to charge recombination. A crystal interface exhibits fewer donor-acceptor contacts with primarily face-on orientation that are more likely to charge-separate because of instant access to the preferred domain.