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A comprehensive study on the relationship between the structure and the photophysical and photovoltaic properties of acceptor–donor–acceptor (A–D–A) type nonfullerene acceptors (NFAs) is of pivotal importance to obtain design guidelines for high-performance organic photovoltaics (OPVs). In this study, we synthesized a D unit, NTT, in which the central benzene core, indacenodithieno[3,2-b]thiophene (IT), is replaced by naphthalene. Notably, NTTIC, an A–D–A type NFA with NTT as the D unit, showed a longer singlet exciton lifetime in the film than the IT-based benchmark NFA, ITIC. When paired with a conjugated polymer donor, PBDB-T, the NTTIC-based OPV device exhibited a higher power conversion efficiency (PCE = 9.95%) than the ITIC-based device (9.71%). Furthermore, due to the larger domain size of NTTIC in PBDB-T:NTTIC, the exciton diffusion (ED) at the donor/acceptor interface and the charge transfer (CT) were slower (14 ps in total) than those of PBDB-T:ITIC (7 ps in total). Nevertheless, the total efficiency of the ED and CT in PBDB-T:NTTIC was similar to that in PBDB-T:ITIC (95%) owing to the longer singlet exciton lifetime of the NTTIC film. These results demonstrate the high potential of naphthalene-cored D units of A–D–A type NFAs to achieve a long singlet exciton lifetime and a resultant high PCE in NFA-based OPVs.