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High boiling point solvent additive, employed during the solution processing of active layer fabrications, impact the efficiency of bulk heterojunction polymer solar cells (PSC) by influencing the morphological of the active layer. The photovoltaic performances of the PSCs based on the donor of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]-dithiophene-2,6-diyl-alt-3-fluoro-2-[(2 ethylhexyl)carbonyl] thieno[3,4-b]thiophene-4,6-diy (PTB7) and the acceptor of [6, 6]-phenyl-C71-butyric-acidmethyl-ester (PCBM) was optimized using 5 vol% high-boiling-point solvent additive of 2-Bromonaphthalene (BN). The optimized air-processed PSC based on PTB7:PCBM (1:1.5 w/w) with 5 vol% BN exhibited a power conversion efficiency of 7.01% with open-circuit voltage ( V ) of 0.731 V, short-circuit current density ( J ) of 13.79 mA cm, and fill factor (FF) of 69.46%. The effects of the additive on photovoltaic performances were illustrated with atomic force microscopy and transmission electron microscope measurements. Our results indicate that the improved efficiency is due to the optimized PTB7/PCBM interpenetrating network and the enhanced absorption of the active layer using the BN as solvent additive. [ABSTRACT FROM AUTHOR]