We report on the synthesis of a new cross-linkable poly(3-hexylthiophene) copolymer, P3HT-Ox10, consisting on regioregular blocks of 3-hexylthiophene and 10% molar of an oxetane-functionalized thiophene comonomer. This copolymer was used in the fabrication of organic photovoltaics (OPVs) upon combination with 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM), either as blends or in bilayer structures. Bilayer devices with a nanostructured donor/acceptor interface were fabricated using a patterning process based on the phase separation in P3HT-Ox10:polystyrene blends. Columnar-grain films of cross-linked P3HT-Ox10 with diameters of ca. 140 nm were thereby obtained. These are the lowest dimensions achieved so far by this approach. Despite the modest power conversion efficiencies achieved, we believe that this easily implemented approach is very effective to prepare OPVs with well-defined morphologies.