The photovoltaic performance of the polymer solar cells (PSCs) based on poly(3-hexylthiophene) (P3HT) as donor and indene-C70 bisadduct (IC70BA) as acceptor was optimized by using 3 vol% high boiling point solvent additive of 1-chloronaphthalene (CN), N-methyl pyrrolidone (NMP), 1,8-octanedithiol (OT) or 1,8-diiodooctane (DIO) without solvent annealing. The optimized PSC based on P3HT:IC70BA (1:1, w/w) with 3 vol% CN and pre-thermal annealing at 150 °C for 10 min, exhibits a high power conversion efficiency (PCE) of 7.40% with Voc of 0.87 V, Jsc of 11.35 mA cm−2 and FF of 75.0%, under the illumination of AM1.5G, 100 mW cm−2. The PCE of 7.40%, the Voc of 0.87 V, and the FF of 75.0% are all the highest values reported in the literature so far for P3HT-based PSCs. The high efficiency is due to the optimized P3HT/IC70BA interpenetrating network and stronger absorption of the active layer by using the additive treatment. Taking into account the advantages of thickness-insensitivity and good reproducibility of the photovoltaic performance of the P3HT-based PSCs as well as the simple device fabrication processes without the need of solvent annealing, the high-efficiency PSCs based on P3HT:IC70BA using CN additive are very promising for future commercialization of PSC devices.