Herein, we present a comparative study on the structural transformations in P3HT/PCBM and P3OT/PCBM blends using octane (C-8) and dodecane (C-12) thiols as solvent additives. Addition of dodecane thiol to the blends promotes needle-like PCBM crystal formation. These crystals are found abundant in P3OT/PCBM and less in P3HT/PCBM blend and are identified by SAXS, DSC and SEM measurements. Using polymer of two different tail lengths, the SAXS results provide the first experimental support on the recent findings on fullerene intercalation around the crystalline boundaries in a semi-crystalline polymer. The UV-Visible measurements reveal that the absorption maximum show an appreciable red-shift and photoluminescence spectra infers that the donor/acceptor interface is mostly retained, upon dodecane thiol addition. These needle-like PCBM crystals obtained by annealing free approach in the blends and their dependence on the alkyl tail length of the polymer, would pave way for more effective design of organic photovoltaic devices. (C) 2013 Elsevier Ltd. All rights reserved.