In this study, we report on the synthesis and self-assembling behavior of a series of monodisperse all pi-conjugated poly(2,5-dihexyloxyp-phenylene)-block-(3-hexylthiophene) (PPP-P3HT) block copolymer system and construct its phase diagram that exhibits diverse nanostructures of predominant PPP nanofibers, PPP-P3HT lamellae, broken lamellae and predominant P3HT nanofibers as a function of the copolymer composition. An order-to-disorder phase transition is also observed for the copolymers at a temperature above the melting point of the predominant blocks, indicating the rod-rod interaction between the corresponding pi-conjugated chains plays a key role in the formation and the stabilization of the observed nanostructures. In particular, we have also found that the incorporation of PPP segments to P3HT to form nanofibrillar structure significantly enhance the self-organization behavior of the P3HT in the copolymer. The improvements in crystallinity and mobility for the copolymers therefore substantially enhance their solar cell performance. The use of this novel material with diverse nanostructures provides a new strategy to enhance photovoltaic performance and shows the potential for use in future optoelectronic applications. (C) 2016 Elsevier B.V. All rights reserved.