(574b) Theoretical Calculations On Fullerene Derivatives

The systematic multiscale of heterogeneous soft matter systems is an area of current research. We are first developing models for a variety of fullerenes used as electron acceptors  in a solar cell's photo-active layer. The donor (typically thiophene based polymers) and acceptor are generally mixed together to produce a bicontinuous percolating network called a bulk heterojunction (BHJ), thereby allowing optimization of both light absorption, which favors thicker devices (>100 nm), and charge-carrier generation and transport, which requires that photogenerated excitons be no further than the exciton diffusion length (~10 nm) from a donor-acceptor interface. The delicate balance between maximizing interfacial area and maintaining percolating pathways for charge transport means that performance is sensitive to the BHJ morphology. But prediction of the active-layer microstructure based on the constituent electron-donor and electron-acceptor phases and the processing conditions remains challenging. Nano-scale morphological information is also often difficult to obtain experimentally. On the other hand, atomistic computer simulations are only feasible to studying systems not much larger than an exciton diffusion length. We overcome this hurdle by developing a coarse-grained (CG) simulation model, in which collections of atoms from an atomistic model are mapped onto a smaller number of "superatoms", of mixtures of the widely used conducting polymer poly(3-hexylthiophene) (P3HT) and various fullerenes. Different fullerene models (with one or more pendant groups) are first developed using quantum chemistry. By comparing the results of atomistic and CG simulations, we demonstrate that the model, parametrized at one temperature and two mixture compositions, accurately reproduces the system structure at other points of the phase diagram. We then use the CG model to characterize the structure and dynamic evolution of the BHJ microstructure as a function of polymer:fullerene mole fraction and polymer chain length for systems approaching the scale of photovoltaic devices. We additionally compare the results to other photoactive polymers.