We synthesized and characterized a series of pyrene units incorporated into benzo[1,2-b:4,5-b']dithiophene-based random copolymers. Concentration-dependent pyrene-driven self-assembly in the copolymers was systematically investigated using measurements of the copolymer optical and electrochemical properties, the hole mobilities extracted from the field-effect transistor and the space charge limited current, and the photovoltaic characteristics. We additionally studied the morphological changes using differential scanning calorimetry (DSC), synchrotron X-ray diffraction analysis, AFM, and TEM. The Py-Py interactions were found to facilitate the formation of fibrous structures. As the Py content increased, multiple Py-Py interactions within a few polymer chains dominated the interchain interactions to produce a more ordered phase but less interconnectivity among many polymer chains. A threshold quantity of the Py moieties enabled the Py-Py interactions to propagate over many polymer chains to form a highly interconnected structure that improved hole transport pathways and, thus, the photovoltaic performances of resulting bulk heterojunction polymer solar cells (5.03 -> 7.52%).