The use of solvent additives has become a successful strategy to control the structural evolution upon film formation in bulk-heterojunction (BHJ) solar cells. Nonetheless, a complete understanding of the additive's role in the phase separation mechanisms and organization of donor and acceptor materials in BHJs is still lacking. In this work we gain further insight about the specific role that a widely used additive, 1,8-octanedithiol (ODT), has in the crystallization of both PCPDTBT and PC71BM, directly after wet film deposition using blade-coating. By in situ X-ray scattering and optical reflectometry, we correlate the additive-driven crystallization with the evolution of film composition from the earliest time of solvent evaporation. It is shown that ODT influences the evolution of both PCPDTBT and PC71BM. ODT leads to prolonged crystallization time during the ODT-drying dominated period corresponding to an overall solvent content (x) of 75 wt % > x > 15 wt % and delays the onset of PC71BM aggregation to x < 20 wt %, being pushed out of the crystalline polymer domains.