We report on the photovoltaic parameters, photophysical properties, optoelectronic properties, self-assembly, and morphology variations in a series of high-performance donor–acceptor (D–A) π-conjugated polymers based on indacenodithiophene and quinoxaline moieties as a function of the number-average molecular weight (Mn), the nature of aryl substituents, and the enlargement of the polymer backbone. One of the most important outcome is that from the three optimization approaches followed to tune the chemical structure toward enhanced photovoltaic performance in bulk heterojunction solar cell devices with the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester as the electron acceptor, the choice of the aryl substituent is the most efficient rational design strategy. Incorporation of thienyl rings as substituents versus phenyl rings accelerates the electron–hole extraction process to the respective electrode, despite the slightly lower recombination lifetime and, thus, improves the electrical p...