We adopted the FeCl3 and Grignard metathesis (McCullough) methods to synthesize three poly(3-hexylthiophene)s (P3HTs) exhibiting different degrees of regioregularity and then blended them with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) to obtain bulk heterojunction phases on the top of an acid-doped polyaniline nanotube (a-PANINT) interfacial layer. From integration of H-1 NMR spectra, we determined that the three P3HTs had head-to-tail coupling contents of 67, 81, and 96%, respectively. The photovoltaic (PV) performance of P3HT:PCBM-based devices fabricated without the a-PANINT interfacial layer increased as the regioregularity of the P3HT increased. The presence of the a-PANINT interfacial layer resulted in improved PV performances of the P3HT:PCBM-based devices. This improvement in the PV performance resulted from the highly conductive, controlled one-dimensional tubular nanoscale morphology of the annealed a-PANINT interfacial layer, which mediated the efficient migration of photogenerated holes to the buffer layer and suppressed exciton recombination.