Multiscale Simulations of Thermal Annealing of P3HT:PCBM Active Layers in Bulk Heterojunctions

Using large-scale coarse-grained molecular dynamics simulations, we have investigated the thermal annealing of poly(3-hexylthiophene) (P3HT) and  phenyl-C61-butyric acid methyl ester (PCBM) blends in the presence of a substrate. The simulations are in agreement with neutron reflectivity (NR) and near edge X-ray fine structure (NEXAFS) experiments and reveal a vertical composition profile of the bulk heterojunction normal to the substrate with enrichment of PCBM near the substrate and air interfaces.  We also demonstrate that the addition of short P3HT chains, as a third component of the blend, can be used to alter the morphology. The short chains are found to efficiently migrate to  the P3HT/PCBM interface.  To model the P3HT/PCBM blend interface, atomistic simulations of 3, 6, or 9 P3HT repeat units in contact with PCBM were performed. These simulations enable the sharpness of the donor/acceptor interface and orientation of thiophene rings at the interface to be examined as a function of P3HT size, and point to the sensitivity of chain orientation on the optimization of interfacial structure.