Naphthalenediimide (NDI) and its π-extended derivatives have been one of the most important building blocks to constitute n-type organic semiconductors. To further explore new materials in the NDI family, an angular-shaped naphthodithiophene diimide (aNDTI) containing an electron-rich angular-shaped naphthodithiophene core fused with two electron-deficient 5-membered maleimide rings was designed and synthesized. The key synthetic step is to furnish the central naphthalene ring of aNDTI by two-step electrophilic and photochemical cyclizations. The optical and electrochemical properties of the new aNDTI and its isomeric lNDTI counterpart were compared theoretically and experimentally. The coplanar aNDTI monomers can serve as a moderate electron-deficient acceptor to prepare two new donor-acceptor copolymers PBM and PFBM. By doping 0.05 wt% PBM and PFBM as nonvolatile polymer additives into the PM6:Y6 blend, the devices achieved enhanced PCEs of 16.1% and 16.0%, respectively, which outperformed the pristine PM6:Y6 device with an original PCE of 15.1%. The improvement of PCEs primarily results from the increase in the FF and Jsc. The GIWAXS measurements reveal that the lamella spacing (dl) of the side-chain interdigitation of PM6 is slightly reduced and Y6 crystallinity is enhanced in the presence of PBM and PFBM additives, leading to enhanced electron (μe) and hole mobility (μh) in the SCLC measurements and higher fill factor (FF) values in the organic photovoltaic (OPV) devices. Furthermore, more efficient photo-induced intermolecular charge transfer between PM6 and Y6 was observed in photoluminescence spectroscopy of the additive-incorporated thin films. This work demonstrates that utilization of polymer donors as nonvolatile additives to mediate morphology is an effective and practical strategy for boosting the efficiency of OPV devices.