Block copolymers have been widely investigated over the past decades for their ability to microphase separate into well-defined nanostructured thin films with tailored physical properties. The aim of the present study is to investigate the thin film properties of rod coil block copolymer/phenyl-C-61-butyric acid methyl ester (PCBM) blends as a function of the blend weight ratio, using a copolymer which is based on a poly(3-hexylthiophene) (P3HT) rod block and poly(4-vinylpyridine) (P4VP) coil block. Atomic force microscopy, transmission electron microscopy and grazing incidence X-ray diffraction analysis are used to study the influence of PCBM on the copolymer self-assembling. UV-visible absorption and photoluminescence spectroscopies as well as field-effect mobility measurements are performed in order to get further insight into the blend optoelectronic properties. It is found that the block copolymer phase-separated morphology and charge carrier mobilities strongly depend on the PCBM loading and thermal annealing. In particular, the results point out that PCBM enhances the block copolymer microphase separation within a narrow range of the polymer:PCBM weight ratio. In addition, clear evidence for PCBM accumulation within the P4VP domains is found by monitoring the P3HT fluorescence and charge carrier mobilities.