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Currently, nearly all high‐efficiency organic photovoltaic devices use donor polymers based on the benzo‐dithiophene (BDT) unit. To diversify the choices of building blocks for high‐performance donor polymers, the use of benzo‐difuran (BDF) units is explored, which can achieve reduced steric hindrance, stronger molecular packing, and tunable energy levels. In previous research, the performance of BDF‐based devices lagged behind those of BDT‐based devices. In this study, a high efficiency (18.4%) is achieved using a BDF‐based polymer donor, which is the highest efficiency reported for BDF donor materials to date. The high efficiency is enabled by a donor polymer (D18‐Fu) and the aid of a solid additive (2‐chloronaphthalene), which is the isomer of the commonly used additive 1‐chloronaphthalene. These results revealed the significant effect of 2‐chloronaphthalene in optimizing the morphology and …