Molecular dynamics simulations of the solvent- and thermal history-dependent structure of the PCBM fullerene derivative

The fullerene derivative PCBM ([6,6]phenyl-C61-butyric acid methyl ester) is one of the best electron acceptors used so far in solution-processed organic photovoltaic devices. The reasons for this success depend partly on its favourable electronic properties, partly on its solubility in common organic solvents and plausibly also on the possibility to optimize its structure and morphology by postdeposition treatments (solvent or thermal annealing). The latter feature is still largely a matter of speculation, as experimentally validated structural models of PCBM molecular organization within the devices are still unavailable. This structural characterization is non-trivial, given that poorly ordered PCBM nanocrystals and amorphous domains appear to often coexist in bulk-heterojunction films based on this system. Here we address some of these issues using molecular dynamics (MD) simulations. Our starting points are the only two published PCBM crystal structures, which were obtained by crystallization from oDCB (ortho-dichlorobenzene) and MCB (monochlorobenzene). Both contain guest molecules of the specific solvent. We simulated their thermal behavior, from room temperature up to their apparent melting points. Additional MD simulations involved model crystals obtained by removing solvent molecules from these co-crystal structures. Models that can apply to the amorphous phase or to nanocrystalline samples have been obtained by cooling molten PCBM, after removing the solvent at different stages in the simulation. Their densities are close to the experimental values and they present a well interconnected network of fullerene moieties, where each of them has an average of seven close neighbours available for charge hopping. Pre- and post-melting structural features such as intermolecular pair distribution functions are discussed in the framework of organic solar cell production and host–guest system dynamics.