The optimization and control of the nanomorphology of thin films used as active layer in bulk heterojunction (BHJ) plastic solar cells is of key importance for a better understanding of the photovoltaic mechanisms and for increasing the device performances. Hereto, solid-state NMR relaxation experiments have been evaluated to describe the film morphology of one of the "work-horse'' systems poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene)/[6, 6]-phenyl-C(61)butyric acid methyl ester (MDMO-PPV/PCBM) in a quantitative way. Attention is focused on the influence of the processing solvent (toluene vs. chlorobenzene), the blend composition, and the casting technique, that is, spin coating versus doctor blading. It is demonstrated that independently of the solvent and casting technique, part of the PCBM becomes phase separated from the mixed phase. Whereas casting from toluene results in the development of well-defined PCBM crystallites, casting from chlorobenzene leads to the formation of PCBM-rich domains that contain substructures of weakly organized PCBM nanoclusters. The amount and physico-chemical state of the phase separated PCBM is quantified by solid-state NMR relaxation times experiments. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1699-1707, 2011