(748b) Coarse-Grained Molecular Dynamics Simulations of Thermal Annealing of P3HT:PCBM Bulk Heterojunctions for Organic Photovoltaic Applications

Molecular dynamics simulation is important in the development of organic photovoltaics because it provides insights on the evolution and the factors affecting the morphology of the bulk heterojunction during the thermal annealing process.   Lee, Pao and Chu [Lee et al., Energy Environ. Sci.,2011, 4, 4124-4132]  recently developed a multiscale approach that allows for the simulation of blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Due to limitations in the system size and simulation duration, however, they were unable to obtain results representative of the entire thermal annealing process. Here we present results obtained on the Titan supercomputer for simulations that are 27 times larger and 10 times longer. The coarse-grained molecular dynamics simulations were performed up to 400 ns and the simulation sizes are comparable to the thickness of the active layer of a bulk heterojunction device (~100 nm). We present results detailing morphological changes in domain size and interfacial area that were not captured by the short simulations by Lee et al. We  characterized and validated the force field with investigation of the P3HT persistence length, PCBM and P3HT packing, and P3HT chain alignment. Additionally, we further studied the effect of the degree of polymerization, N of P3HT on the morphology and investigated the miscibility of P3HT and PCBM.