The impact of the additive 1,8-diiodooctane on the morphology of bulk-heterojunction solar cells based on the systems P3HT:PC₇₁BM, PTB7:PC₇₁BM and PTB7-Th:PC₇₁BM is studied using a combination of Small Angle Neutron Scattering (SANS) and Atomic Force Microscopy (AFM). The results clearly show that while in the P3HT:PC₇₁BM system, the additive DIO promotes a slight coarsening of the phase domains (type I additive), in the systems PTB7:PC₇₁BM and PTB7-Th:PC₇₁BM, DIO promotes a large decrease in the size of the phase domains (type II additive). SANS is demonstrated as being particularly useful at detecting the minor morphological changes observed in the P3HT:PC₇₁BM system, which can be hardly seen in AFM. This work illustrates how SANS complements AFM and both techniques when used together provide a deeper insight into the nanoscale structure in thin organic photovoltaic (OPV) device films.