Effect of Terminal-Group Halogenation of Naphthalene-Based Nonfullerene Acceptors on Their Film Structure and Photophysical and Photovoltaic Properties

Terminal-group halogenation is an effective strategy for tuning energy levels, improving light-harvesting ability, and enhancing the intermolecular stacking of nonfullerene acceptors (NFAs) for organic photovoltaic (OPV) devices. Understanding the influence of different halogen atoms on their film structure and photophysical and photovoltaic properties is crucial for designing NFAs. To address this issue, three acceptor–donor–acceptor (A–D–A)-type NFAs, named NTTIC-F, NTTIC-Cl, and NTTIC, were designed. All three NFAs consisted of a naphthalene-containing fused-ring as a D core unit and dihalogenated or nonhalogenated 1,1-dicyanomethylene-3-indanone groups as terminal A units. NTTIC-Cl exhibited broadened absorption and downshifted energy levels compared to NTTIC-F and NTTIC. Meanwhile, the singlet exciton lifetimes in solution increased in the order of NTTIC < NTTIC-F < NTTIC-Cl, but these were shortened in the pristine films and rather comparable. From grazing-incidence wide-angle X-ray scattering measurements, the tendency of NTTIC-F < NTTIC < NTTIC-Cl was found for the formation of the face-on-oriented packing structures. To the best of our knowledge, this is the first observation of a change in the molecular orientation of NFAs by the type of the halogenation atom at the terminal A unit. When blended with J71 as a conjugated polymer donor, the NTTIC-Cl-based OPV device showed a higher short-circuit current density (J_SC, 20.5 mA cm^–2) than the NTTIC-F- and NTTIC-based devices (19.6 and 15.6 mA cm^–2, respectively). Overall, the power conversion efficiencies of the NTTIC-F- and NTTIC-Cl-based devices were similar (10.6 and 10.5%, respectively) but higher than that of the NTTIC-based device. The former resulted from the higher open-circuit voltage (V_OC, 0.812 V) and fill factor (0.666) of the NTTIC-F-based device than those of the NTTIC-Cl-based device (0.793 V and 0.646, respectively).