Development of a phenanthrodithiophene‐difluorobenzoxadiazole copolymer exhibiting high open‐circuit voltage in organic solar cells

A phenanthrodithiophene (PDT)‐difluorobenzoxadiazole (DFBO) copolymer, P‐PDT‐DFBO , was synthesized and characterized. Replacing a thiadiazole with an oxadiazole ring gives the synthesized polymer a highest occupied molecular orbital (HOMO) about 0.1 V lower, and lowest unoccupied molecular orbital energy levels lower than those of its benzothiadiazole (BT) counterpart, due to the more electron‐deficient oxadiazole. Furthermore, since oxadiazole has a larger dipole moment than BT, P‐PDT‐DFBO exhibits greater aggregation strength than previously reported for P‐PDT‐DFBT . The low‐lying HOMO level of P‐PDT‐DFBO gave about 0.1 V higher open‐circuit voltage ( V oc ), yielding over 0.9 V in a fabricated solar cell. From grazing incidence wide‐angle X‐ray diffraction analysis, P‐PDT‐DFBO formed a favorable face‐on orientation in both neat and blended films, indicating that the incorporation of an oxadiazole moiety can enhance V oc without any orientation change in the solid state. However, a P‐PDT‐DFBO ‐based cell exhibited significantly lower J sc and FF, and thus less power conversion efficiency, not >4.43%, due to its lower hole mobility than P‐PDT‐DFBT . One possible reason for poor performance may be the low crystallinity of P‐PDT‐DFBO in blended film. This may be caused by its strong aggregation tendency, leading to fast crystallization into a semiamorphous structure or to interference with the construction of long‐range ordered structure. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56 , 2646–2655