From an extensive analysis of experimental data, it has been hypothesized recently that the performance of organic photovoltaics devices strongly depends on the intermolecular arrangements at the donor polymer/fullerene interfaces (K.R. Graham et al., J. Amer. Chem. Soc. 136 (3014) 9608). In this presentation, we seek to provide an understanding of the phenomenon by discussing the results of a combined electronic-structure calculations and molecular-dynamics simulations on polymer/fullerene bulk heterojunctions, both for representative systems reported in the literature and new systems synthesized in our Center. In particular, we examine: (i) the propensity of the fullerene molecules to dock preferentially on top of the electron-poor moiety or electron-rich moiety of the polymer, as a function of the nature and location of the polymer side chains; and (ii) the impact the packing arrangement has on the interfacial electronic structure. This work has been funded by King Abdullah University of Science and Technology in the framework of its Solar & Photovoltaics Engineering Research Center (SPERC) and its Collaborative Research Grant Program (Award CRG3-62140391).