Effect of substitution positions of alkyl side chains in phenanthrodithiophene–isoindigo copolymers: The enhancement of crystallinity and control of molecular orders

The synthesis, characterization, and solar cell application of newly developed two semiconducting polymers containing phenanthro[1,2-b:8,7-b′]dithiophene (PDT) and an isoindigo (IID) unit are described. In addition, a relationship between substitution position of side chains and molecular weights of the polymers and their solar cell performance are also discussed. Because of the installation of alkyl side chains onto sterically less hindered positions, PDT-IID copolymers 12OD-2 and 8OD-2 have stronger intermolecular interaction than that of the previously reported copolymer 12OD. In low-M_n polymers 12OD-2 and 8OD-2 formed high-crystalline thin film with higher face-on ratio than that of 12OD, but their unsuitable large-scale phase separation suppressed their efficient photocurrent generation, leading to poor PCE of 2–3%. However, the surface morphology of 12OD-2 and 8OD-2 blended films are drastically improved by increasing M_n, which leads to the enhancement of J_sc and higher PCE of up to 4.3%. However, high-M_n polymers 12OD-2 and 8OD-2 formed high-crystalline film with about 10–15% lower face-on ratio than that of high-M_n polymer 12OD, leading to poor hole transporting ability, and thus lower J_sc and PCE. From this result, too much strong intermolecular interaction promotes the formation of unsuitable edge-on orientation in blended films.