Derivatization and diffusive motion of molecular fullerenes: Ab initio and atomistic simulations

Using first principles density functional theory in combination with the nonequilibrium Green's function formalism, we study the effect of derivatization on the electronic and transport properties of C₆₀ fullerene. As a typical example, we consider [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), which forms one of the most efficient organic photovoltaic materials in combination with electron donating polymers. Extra peaks are observed in the density of states (DOS) due to the formation of new electronic states localized at/near the attached molecule. Despite such peculiar behavior in the DOS of an isolated molecule, derivatization does not have a pronounced effect on the electronic transport properties of the fullerene molecular junctions. Both C₆₀ and PCBM show the same response to finite voltage biasing with new features in the transmission spectrum due to voltage induced delocalization of some electronic states. We also study the diffusive motion of molecular fullerenes in ethanol solvent and inside poly(3-hexylthiophene) lamella using reactive molecular dynamics simulations. We found that the mobility of the fullerene reduces considerably due to derivatization; the diffusion coefficient of C₆₀ is an order of magnitude larger than the one for PCBM.