Polymer solar cells (PSCs) based on fullerene derivatives often require additives to optimize active layer morphology. Here, the novel additive 1-naphthalenethiol (SH-na) is proposed for processing the PSC active layer of PTB7:PC71 BM. Spin-casting with SH-na as additive achieves a power conversion efficiency (PCE) of 7.3%, compared to 6.7% for preparations containing the conventional 1,8-diiodooctane additive. Dipping of the active layer into a methanol solution of critical SH-na concentration increases the PCE further to 8.75%. This is mainly due to an improved open-circuit voltage (from 0.72 to 0.79 V) together with a high achieved fill factor of 0.70. The improved PCE is correlated to the morphology optimization according to measurements of grazing incidence small/wide-angle X-ray scattering, neutron reflectivity, atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The integrated results suggest that the halogen-free additive SH-na can form hydrogen bonds with both PTB7 and PC71 BM, resulting in substantially improved PTB7 crystallization and multi-length-scale PC71 BM dispersion for appropriate aggregation and networks. The subsequent dipping treatment with SH-na further modifies the active layer morphology for a more PC71 BM-enriched surface and better PC71 BM networks in the bulk film for an optimized electron-to-hole mobility ratio of 2.04, hence resulting in improved device performance.