New D-A1–D-A2-Type Regular Terpolymers Containing Benzothiadiazole and Benzotrithiophene Acceptor Units for Photovoltaic Application

Two novel regular terpolymers that are of D-A1–D-A2 type and contain benzothiadiazole and 2,5-dibromo-8-dodecanoylbenzo[1,2-b:3,4-b′:5,6-d″]trithiophene (P1) or 2,8-dibromo-5-dodecanoylbenzene[1,2-b:3,4-b′:5,6-d″]trithiophene (P2) acceptor units with the same thiophene donor were synthesized through Stille coupling, and their optical and electrochemical properties were investigated. The highest occupied molecular orbital (HOMO) and lowest unoccupied (LUMO) molecular orbital energy levels of these terpolymers indicate that there is sufficient LUMO offset with PCBM for efficient exciton dissociation, and their deeper HOMO levels ensure the high open-circuit voltage for the resultant bulk heterojunction solar cells. Measurements on the solar cell devices also confirm that compared to those based on P2the devices based on P1possess a higher short-circuit photocurrent (Jsc) as well as a higher fill factor (FF), which is attributed to the lower bandgap and higher hole mobility for P1, whereas the Vocis higher for the devices that are based on P2, which may be a result of P2 having a lower HOMO energy level than P1. The optimized polymer solar cells fabricated using P1:PC71BM (DIO/CF) and P2:PC71BM (CF/DIO) for the active layers showed a PCE of 7.19% and 6.34%, respectively. Atomic force microscopy (AFM) images of P1:PC71BM blend films show that they exhibit more suitable morphology with favorable interpenetrating networks, which favors high Jscand FF. Moreover, P1exhibits a more crystalline nature than P2that also favors the charge transport. This may be a result of better molecular packing, more distinct phase separation of the blended films, as well as a reduction of charge recombination.