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

Abstract

ABSTRACTA 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 (Voc), 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 Voc without any orientation change in the solid state. However, a P‐PDT‐DFBO‐based cell exhibited significantly lower Jsc 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