The influence of octanedithiol (ODT) on the film morphology of PCPDTBT:PCBM (1:2) blends processed from solutions in chlorobenzene was studied by chemical shift selective carbon-13 solid-state NMR relaxation experiments. A close contact between PCBM and PCPDTBT is demonstrated for PCPDTBT:PCBM films processed with as well as without ODT, i.e. the dimensions of the substructures in the PCBM rich and polymer rich domains are smaller than 10 nm. The PCBM organization is shown to be reduced towards very small and weakly organized clusters. The main difference of processing the active film in the presence of ODT can be found in the ability of the polymer to improve its chain packing towards a state that closely resembles this of pure PCPDTBT. This difference in physical interaction between the polymer chains can explain the improved power conversion efficiency and the EQE red-shift of solar cells processed with ODT. (C) 2011 Elsevier B.V. All rights reserved.