Organic photovoltaics (OPVs) are a class of solar cells that harvest photogenerated excitations ('excitons') and split them into free charges to generate electrical power. A highly promising but poorly understood OPV architecture is the low-donor bulk heterojunction, in which a small volume of an electron donor is distributed within a host matrix of an electron acceptor, such as C60 fullerene, to facilitate exciton splitting. Based on recent spectroscopic and x-ray scattering results, we propose using small-angle neutron scattering to probe a series of films in which the donor concentration and crystallisability are varied. The observed effects on the spatial distribution of the donor, and its impact on the nanoscale morphology of the host matrix, will help shed light on their underlying operational mechanism, and highlight new routes for higher efficiency OPVs.