Diblock copolymers bearing a triphenylene (TP) discotic liquid crystals moiety, poly(3-hexylthiophene)-block-poly[3-(10-(2,3,6,7,10-pentakis(hexyloxy)triphenylen)-decyloxy)thiophene] (P3HT-b-P3TPT), was successfully synthesized by Grignard metathesis polymerization. The self-assembled nanowire structures of these diblock copolymers have been investigated by atomic force microscopy and transmission electron microscopy. The domain size and crystallinity of the nanostructures can be easily controlled by tuning the P3HT/P3TPT block ratio and by employing different annealing processes such as thermal and solvent annealing. The results of X-ray diffraction indicate that both intermolecular interactions and mesogen packing are essential for the formation of nanostructures in the diblock copolymers. Although the block ratio of P3HT and P3TPT comes to 9:1 and the copolymer undergoes solvent annealing followed by thermal treatment, an optimal crystalline nanowire with a size of 16.9 nm is formed. In addition, solar cells based on these copolymers as electron donors in combination with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or N,N'-di(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylbisimide (PDI) as electron acceptors have been constructed, and the effect of the nanomorphology on device performance has been investigated.