Polymer solar cells (PSCs) with poly(3-hexylthiophene) (P3HT) as a donor, an indene-C70 bisadduct (IC70BA) as an acceptor, a layer of indium tin oxide modified by MoO3 as a positive electrode, and Ca/Al as a negative electrode are presented. The photovoltaic performance of the PSCs was optimized by controlling spin-coating time (solvent annealing time) and thermal annealing, and the effect of the spin-coating times on absorption spectra, X-ray diffraction patterns, and transmission electron microscopy images of P3HT/IC70BA blend films were systematically investigated. Optimized PSCs were obtained from P3HT/IC70BA (1:1, w/w), which exhibited a high power conversion efficiency of 6.68%. The excellent performance of the PSCs is attributed to the higher crystallinity of P3HT and better a donor–acceptor interpenetrating network of the active layer prepared under the optimized conditions. In addition, PSCs with a poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) buffer layer under the same optimized conditions showed a PCE of 6.20%. The results indicate that the MoO3 buffer layer in the PSCs based on P3HT/IC70BA is superior to that of the PEDOT:PSS buffer layer, not only showing a higher device stability but also resulting in a better photovoltaic performance of the PSCs.