Fluorination effect of benzo[c][1,2,5]thiadiazole-alt-oligothiophene-based copolymers involving all straight flexible side chain in photovoltaic application

Abstract Two benzo[c][1,2,5]thiadiazole (BT)-based alternating conjugated copolymers (CPs) with donor-π-acceptor (D-π-A) structure, namely, P2T-DTBT and P2T-DTFBT, where 3,3′-didodecyl-2,2′-bithiophene (2T), 3-hexadecylthiophene and BT and/or 5,6-difluoro- benzothiadiazole (FBT) were correspondingly utilized as D, π-bridge and A units, were prepared and fully characterized. Due to selecting the oligothiophene and BT and/or FBT subunits to build the polymer backbone, the obtained copolymers bearing all straight side chains elucidated the wide light absorption between 300 and 700 or 800 nm and deep energy level of highest occupied molecular orbital (EHOMO) from −5.28 to −5.35 eV. The little impact on thermostability, light absorption and aggregation in chlorobenzene but pronounced influence on light absorption and aggregation in film were observed after introducing fluorine into BT in P2T-DTFBT. It has been exhibited that the fluorination could gain 0.17 eV reduced optical band gap and 0.07 eV descended EHOMO. Also, the more ordered and compacted film stacking were formed, leading to the balanced and increased more hole/electron mobilities, and more advantageous morphology of the active layers. As a consequence, P2T-DTFBT-based device afforded the 88.44% increased PCE from 2.25% to 4.24%, with the 9.65% increased VOC, 41.99% enhanced JSC and 23.91% elevated FF than that of P2T-DTBT-based device. These experimental results demonstrated the fluorination was a simple and effective tactic for modulating the molecular structure and elevating the photovoltaic performance.